StarCraft/Counters

Probe
This will almost never be used for military purposes, other than scouting, early harassment, or the dreaded "Cannon Rush" strategy. Each race can put up towers (Bunkers, Photon Cannons, etc.) to discourage Probe espionage.

Zealot
The Zealot is significantly more powerful than its Zergling and Marine counterparts, but it cannot attack air units. They remain useful into the late game, especially with their speed upgrade.
 * Zerg: Swarming with Zerglings or using "Hydralisk Dancing" techniques are generally effective. Lurkers work well when they are available, as does any air unit.
 * Terran: Firebats(OUTDATED) with Medic support work well, and Vultures are even better (remember to micromanage them for maximum efficiency). Again, any air unit with a ground attack is effective.
 * Protoss: Zealots of your own with Psionic Storm or Reaver support work well. Scouts, Carriers, and undetected Dark Templar can attack with impunity.

Dragoon
The Dragoon is a staple of most Protoss forces, often seen backing up Zealots and Reavers.
 * Zerg: Swarms of Zerglings and Mutalisks are good counters because the Dragoon deals less damage to "small" units. The Defiler's Dark Swarm and the Queen's Spawn Broodling spells are good choices as well.
 * Terran: Marines and Medics, with their small size and full damage attack, are the best counter until Siege Tanks come into play.
 * Protoss: Zealots can defeat an equal Dragoon force if left unchecked. Reavers and Psionic Storm are good support choices as well.

High Templar
The High Templar is a spellcaster with no attack, slow speed, and powerful spells. Any time it is unsupported and in the open is a good time to attack it.
 * Zerg: Small packs of Zerglings can take out lone Templar easily. Queens with Spawn Broodling are effective for picking off Templar in battle, and Ultralisks have enough hit points to absorb a few Psionic Storms. If your Mutalisk control is good, consider using these to "snipe" Templar before they can inflict damage.
 * Terran: The Science Vessel's Irradiate is a good choice, though unstacked Battlecruisers are strong enough to take Psionic Storm attacks without dying. However, because the Terran mechanical build is innately vulnerable to Psionic Storm (Tanks are often immobile, and Goliaths are weak and sluggish) consider a squad of Vultures to fight this threat.
 * Protoss: The Dark Archon's Feedback spell works well, and Reavers can strike a far-off Templar with their long range attack. Dark Templar sneak attacks can also discourage Templar strikes.

Dark Templar
The main threat with this specialty unit is in its permanent cloaking effect. Once detected, its main advantage is neutralized (though it still boasts a powerful attack).
 * Zerg: Keep Spore Colonies (Sunken Colonies cannot detect) and Overlords near at all times. The Queen's ensnare can also reveal cloaked units. Once detected Hydralisks or Mutalisks can make short work of the unit.
 * Terran: Keep Missile Turrets and Science Vessels nearby at all times (additionally, the Vessel's Irradiate Spell works well). Comsat Stations are a short-term solution, but an experienced Dark Templar user can work around them.

If you plan on going mech and you're worried about a Dark Templar rush, instead of creating a Comsat right away, you can use Spider Mines for the time being. Just make sure you put them at your base entrance so that the Mines will not miss.
 * Protoss: Build Photon Cannons around your base and bring Observers with your attack force.

Archon
The Archon has a tremendous (350 point) shield and a brutal splash melee attack, though it has little hit points. Shields take full damage from all enemy attacks.
 * Zerg: Use the Guardians' long range to take them out from afar. Try to keep the Archon at a distance whenever possible, and try not to use Zerglings or Mutalisks. Hydralisks, a good number of which can focus fire for great damage, are a good bet.
 * Terran: Obviously, the Science Vessel and its shield draining EMP are a perfect counter, though be ready to follow up with regular units to deal the final 10 damage (do note that EMP has a smaller radius than you may think, though, so if there are multiple Archons spaced properly this may not be very effective). If Science Vessels are not available, Vultures with some micro can destroy Archons quickly as they do full damage to the Archon's 350 shield.
 * Protoss: Reavers and Carriers can deal heavy damage to Archons from afar, but be ready to support these units should the Archon get close. A large number of Dragoons also work well.

Dark Archon
The Dark Archon is a dangerous spellcaster, but, like the Archon, has few hit points and high shields. Tempt the Dark Archon to mind control a unit, draining all its shields. Then, with 25 HP, it will be as good as dead.
 * Zerg: Maelstrom is always a threat, stunning all biological units in a certain area for 10 seconds. Spread out your units to avoid the worst, and try and kill it quickly. Spawn Broodling will not work on the Dark Archon as it did for the Templar (and the Dark Archon's Feedback can destroy Queens easily). Try to keep a careful eye on any Ultralisks you're using; if these get Mind Controlled you're in trouble.
 * Terran: Tanks' range is three times greater than that of Mind Control, and the TvP matchup usually doesn't yield Terran units that are worth using the ability for anyway. Using many mechanical units works. The Yamato Gun also destroys a Dark Archon in one hit.
 * Protoss: Use Reavers for their range and power. Your own Dark Archons with Feedback can work well.

Reaver
Reavers are good at taking out zerg and terran swarms, as well as all grouped units (particularly gatherer units). They also have good impact against armor, such as targets like bunkers, sunken colonies, photon cannons, or vehicles. They are good en masse, but cannot target air units so try to use that to your advantage. They are also used in quickly taking out small outposts or enemy gatherers by dropping them behind defenses. Cheap units beware. Creating a few mutalisks will help stymie a reaver problem. Reavers can't fire on air units, but are commonly transported with shuttles. You can also use the mutalisks against the shuttles if they are in retreat, but if the enemy strategy is to drop reavers, build some scourge to deal with their transports. If you are quick, you can use the ghost to lock them. Locking units is very useful. When they are already locked, you can strike it with your marines or firebats. But make sure your Ghost is cloaked so that the reaver can't fire on him. Dark Templars are good against reavers. They are stealth units that deals heavy damage. If you're not playing Brood War, use High Templar with Psi Storm, scouts, or carriers.
 * Zerg
 * Terran
 * Protoss

Scout
Scouts can attack air and ground, and are considered the best fighter craft, but their missiles are explosive and lasers deal 8. Most ground to air units will work against them when used in numbers, but multiple scouts can overpower static defenses and some ground units.
 * Zerg: There are two reasons why Mutalisks work: Scouts will be out numbered, and missiles deal only 14 damage to Mutalisks. Hydras with or without Dark Swarm can also be particularly effective, but multiple Hydralisks are needed for facing Scouts.
 * Terran: Goliaths can destroy Scouts easily. Two or three Goliaths (preferably with the range upgrade) can outmatch a Scout. The air is a different story. The Scout outnumbers and beats Battlecruisers and one Scout may even beat two Wraiths. However, one Yamato hit or cloaking the Wraiths will overpower the Scout. Valkyries are the most effective air counter to groups of scouts.
 * Protoss: Scouts are easily countered by Dragoons, and maybe High Templar with Psionic Storm. To confront a Scout in the air, outnumber it with Corsairs, or use at least two or three Carriers.

Corsair
The Corsair is a fast air unit with a rapid, splash dealing air-to-air attack. These units have a special ability called the Disruption Web which cause enemy ground forces and structures to be unable to attack as long they're under the web's effect.


 * Zerg: The Devourers, with their heavy armor and Acid Spores, are best suited to countering Corsairs en masse. However, Corsairs will often show up extremely early in the game in a Corsair-Reaver or Corsair into Dark Templar combination, before Hive tech is feasible. In these circumstances, you must choose from either multiple Hydralisks, a Spore Colony, or Scourge to prevent Overlords from being killed. Floating all Overlords into a single cluster around a Spore Colony will render most Corsair harassment by small numbers of Corsairs ineffective, but will remove detection from expansions making them vulnerable to Dark Templars.
 * Terran: Battlecruisers and Valkyries, with their heavy armor, work best. If unavailable use Goliaths.
 * Protoss: Corsairs rip through Interceptors fast, so Carriers are a poor choice. In small battles where the Corsair's splash has less effect, Scouts can defeat Corsairs. Stick with Corsairs of your own, or try using a Dark Archon's Feedback.

Carrier
The Carrier is known for its 4 base armor, and its ability to run and attack at the same time. Heavy armor works well against them, as Intereptors only do 6 damage each.
 * Zerg: The Scourge are your best bet with 110 damage each. The Carrier can barely fight back before it's destroyed. Also, Devourers weaken its armor, and stand well against Interceptors.
 * Terran: Yamato Gun if you have it, but cloaked Wraiths if they don't have Detection work better. Ghost Lockdown is also very effective. However, the most effective build against mass Carriers is mass Goliaths; on maps like Katrina, where Protoss users almost always go Carriers against Terran, you should get two Armories up early to research +2 attack and +1 defense for ground units and pump Goliaths. Protoss will produce Dragoons to counter the Goliaths, so produce some Tanks as well. The rest is a balancing match between the Carrier+Dragoon force and the Goliath+Siege Tank force.
 * Protoss: Scouts can work, but they are generally not recommended. Lots of Dragoons, and even better, High Templar with Psionic Storm. It deals a massive 112 damage, and the Carrier is slow, meaning it will take much of the damage. Also a group of Corsairs can easily destroy a fleet of Carriers since they do splash and will most likely devastate the Interceptors as well.

Arbiter
The Arbiter cloaks every friendly unit around it, but cannot cloak itself. Also, 10 damage explosive is not a lot.


 * Zerg: Probably any anti-air unit, particularly Scourge. Just keep in mind, large groups of anything can be Stasised, and large numbers of friendly units can be summoned for an offensive Recall. All in all Arbiters usually are not seen in PvZ matches.


 * Terran: If you are having trouble with Arbiters, the EMP Shockwave of the Science Vessel (upgraded at Science Facility) will drain all energy and shields within the blast radius. Scout the Arbiters with Comsat and use this to effectively render them useless. Goliaths and Missile Turrets also make short work of an unprotected Arbiter, so dot your base with one or both to prevent surprise Recalls.


 * Protoss: You can Mind Control the Arbiter using a Dark Archon, or just keep an Observer nearby to render their cloak useless.

Shuttle

 * Zerg: The best way to counter a Shuttle is to keep Scourge and Spore Colonies around to prevent drops. Also, Lurkers will devastate any ground units that are actually dropped.


 * Terran: Missile Turrets will destroy the Shuttle easily. In case the Shuttle makes drops before it dies, keep a few Tanks in Siege Mode to immidiately harass the dropped unit(s), most likely a Reaver.


 * Protoss: Build Photon Cannons to defend against drops, and use Corsairs to patrol around your base.

Drone
The Drone is the worker for Zerg. It is less common to see them as scouting units because a Zerg player almost always has numerous Overlords.


 * Zerg: Mutalisks are great for worker harassment, because they can hit and run fast, as well as damage more than one worker at a time with their unique splash attack.
 * Terran: In late game, Irradiate is good for taking out worker lines. Stacked cloaked Wraiths work well if the opponent is lax on their cloaking detection.
 * Protoss: The High Templar's Psionic Storm is an effective weapon that can kill nearly all of a player's workers with little warning. Reavers also make great worker killers because of their splash damage.

Infested Terran
The Infested Terran is probably the best suicide attacker in the game. Able to deal 500 points of damage, these units can kill any enemy ground units in a one on one fight. Their attacks also do splash damage, so its best used against groups of enemies.


 * Terran: Use large groups of Siege Tanks in Siege mode to stop these suicide attackers from causing harm to your units. Bunkers can be destroyed in one hit from Infested Terrans if they are not used properly.
 * Zerg: Groups of Guardians and Hydralisks are pretty effective at stopping an Infested Terran from causing trouble.
 * Protoss: Most ground units will not work unless you even them out to minimize the damage done by the infested terran doing this and using dragoons would make it hard for the infested terran to cause damage since it would most likely die before it reaches... use an observer to detect and burrowed infested terrans.


 * All: Try to keep Anti air around your base to prevent a drop of these monsters because they can easily destroy your entire workforce,(SCV,s/Probes/Drones)even important tech buildings.

Overlord
The Overlord is most encountered scouting through the base to check on the enemies' tech. It is slow moving, and has no attack, but the Zerg player will often have plenty to spare.


 * Zerg: Mutalisks and Hydralisks are the best tools for chasing scouting Overlords away, as they are the best anti-air units in the Zerg arsenal. Two Scourges might also work as well.
 * Terran: Wraiths, Goliaths, Valkyries, and Marines are all good tools for killing/warding away scouting Overlords, though Valkyries are particularly well-suited to destroying groups of Overlords with their splash damage.
 * Protoss: Corsairs are generally most effective (especially against groups, like the Valkyrie), though Scouts and Dragoons are ample substitutes.

Zergling
The Zergling is the most basic Zerg unit, but don't underestimate its usefulness. At only 25 minerals and half a supply point per Zergling, a swarm of them can be amassed fairly quickly, and used for an early-game rush.


 * Terran: Groups of terran marines and firebats with medics can easily hold off zerglings its probably best to get them in a thin area where thay cant use their numbers to their advantage using irradiate on tightly packed zerglings works well also.Any air units works well too.


 * Zerg: Counter them with equal or larger numbers of Zerglings and a few Hydras and Lurkers, or use Mutalisks and Guardians. Alternatively, try using a Defiler's Plague on a large group and bring in Hydras.


 * Protoss: Zealots match up well, especially with upgraded weapons, but don't use Dragoons. Also, use a Templar's Psionic Storm and Reavers when they are available.

Hydralisk
The Hydralisk is the bread-and-butter Zerg unit, similar in function to the Marine but stronger and with an explosive attack.


 * Terran: Use Marines and Medics; the Hydralisk does half damage to them, and they do full damage to it. Later on, large numbers of Siege Tanks can hold them off, and groups of Battlecruisers work very well.Using irradiate on tightly packed groups will work well also
 * Zerg: Counter them with masses of Zerglings, with the speed upgrade preferably. Lurkers can also work very well.Guardians are very effective also use the terrain and your range to your advantage
 * Protoss: Use Zealots with the speed upgrade, Reavers, or a Templar's Psionic Storm.Mael storm is very effectice also.

Ultralisk

 * Terran: The only real way to counter ultralist with the terrans is to use sheer fire power target them with yamato guns or use stim pack to overwhelm them use your siege tanks to target them and quickly kill it.


 * Zerg: Overwhelming numbers like zerglings can easily kill a ultralisk use broodling or you can try to sneak a plague on them before they enter battle.


 * Protoss: Arcrons are probably the best counter against since they match up pretty well using a good number of zealots or just using a dark archron and posseses it.

Lurker

 * Terran: Using Siege Tanks and air units is probably your best bet. Always keep some sort of detection around when fighting Lurkers; using a Science Vessel's Irradiate ability also works really well. If you are using a biological build (which most players do when playing against Zerg), having a few Vessels around to complement a large army of M&Ms usually negates the Lurker threat almost completely. Just remember to scout with Vessels to avoid falling into Stop-Lurker traps.


 * Zerg: Ultralisks are really effective because they can take many hits. Using air units and Using Spawn Broodling on them is also effective.


 * Protoss: If there are only a handful of Lurkers, Archons can actually take them out without detection with their powerful splash damage. If there are more, use a combination of Zealots and Archons. Dragoons with the range upgrade can hit Lurkers without being in any danger, but are vulnerable to other Zerg ground units if used exclusively. Air units are effective as well, but perhaps the best method is the High Templar's Psionic Storm; if you know where they are, a good Storm or two will weaken them near to the point of death, or kill them outright.

Mutalisk
The Mutalisk has the distinction of being classified as a "small" unit, meaning it takes 1/2 damage from explosive weapons, which are most anti-air weapons. Therefore it can be extremely useful, even later in the game. It also has a more balanced air/ground attack than its counterparts, and is very fast and relatively cheap.


 * Terran: Marines with the range upgrade and stimpacks, backed up by Medics, go through Mutalisks like a hot knife through butter. If you are forced to use Goliaths or Wraiths, make sure you have the range and cloaking upgrades, respectively. Valkyries do great damage to Mutalisk groups, but are hardly useful outside of this role, so don't go crazy with them.
 * Zerg: Try using a Defiler's Plague on stacked Mutalisk groups, and bringing in your own, or a group of Hydralisks with the Range upgrade.
 * Protoss: Groups of Corsairs work well, as does a Templar's Psionic Storm. If the enemy has large numbers of stacked Mutalisks, use an Arbiter's Stasis Field to discourage this kind of behavior.

Devourer
The Devourer is the most dangerous anti-air unit in Brood War. Its attack has a splash radius and causes enemies to take more damage and have a slow rate of fire.


 * Terran: Marines and Goliaths are the best way to deal with a Devourer and a Yamato Cannon kills a Devourer in one hit.
 * Zerg: the Hydralisk is the best way to deal with them but Scourges might work just as well.
 * Protoss: Carriers do not get affected by its Acid Spores, due to the deployment of eight Interceptors to attack their targets. Archons and the Dragoon are perhaps the best way to deal with the Devourer.

Guardian
The Guardian has a vast range, and can often even take down other units with impunity (since they simply can't reach the Guardian), and can even take out Terran Missile Turrets without fear of being hit. However, they are slow and very vulnerable to air units unless you have Devourers, Hydralisks, or Mutalisks on your side with them.
 * Terran: Your best bet are Wraiths and Valkyries, if they have no anti-air with them. However, if they do, then bring Goliaths with a missile range upgrade. A science vessel with Irradiate (or even Valkyries) can do well if the units are heavily stacked.
 * Zerg: Dark Swarm and range-upgraded Hydralisks.
 * Protoss: Corsairs will easily take down these units, even if they have a couple of Devourers. Beware: you must micromanage your Corsairs to keep them spread out if they have many Devourers; they will slow your Corsairs down to the point of uselessness. Alternatively, a combination of High Templars with Psionic Storm and Archons will take care of them quickly if they're stacked.

Scourge
They're flying Infested Terrans, without splash. Best used against Carriers, since four or five can take them down before they even know what's happening. If the opponent is cloaking a Zealot army with an Arbiter, these can also give you a little help (even though it shouldn't matter because you have Overlords). Not much use in Zerg vs. Terran or Zerg vs. Zerg, since both of these races will usually have other units that will kill your Scourges too fast. However, it may be helpful to give these little guys a couple of points into fly carapace – they can take a couple more hits that way, and a couple more hits is a couple more seconds to get to the target.


 * Terran: Wraiths, Goliaths, or Valkyries will annihilate Scourges pretty quickly. A lone Battlecruiser may succumb to a few of these, though.
 * Zerg: Don't send out an army of Guardians or Devourers without Mutalisks or Hydralisks, and you'll be fine. Alternatively, more Scourges is also an answer.
 * Protoss: A few Scouts or Dragoons will kill Scourges pretty quickly – basically any anti-air with a higher attack than 10. A group of a few will kill them pretty easily. 6+ Corsairs can kill off all incoming Scourge.

SCV

 * Terran: Vultures and Wraiths are great for SCV harassment.


 * Zerg: Mutalisk and Lurker drops are very effective for SCV harassment.


 * Protoss: High Templars with the Psi Storm ability and Reaver drops are very effective for SCV harassment.

Marine

 * Terran: Using marines can counter other marines have medics with your marines to make them more effective siege tanks are also very effective against marines.


 * Zerg: Groups of hydralisks and zerglings are also very effective lurkers work very well also.


 * Protoss: Use zealots and dragoons to counter marines till later in the game start using High Templars with psi storm and reavers.

Firebat

 * Terran: Most ranged units are effective and air units goliaths and tanks work very well against firebats as well.


 * Zerg: Groups of hydralisk can take out firebats easily ultralisk, lurkers and air units are very effective agaisnt firebats.


 * Protoss: Dragoons and and air units are very effective against firebats you can also use psi storm and reavers to destroy them easily.

Medic

 * Terran: Medic normally isnt a big deal but if is use the science vessel ability irradiate to target medics but beware if the player has another medic nearby will most likely restore the irradiate on the medic and use long range units like siege tanks to attack them and kill them quickly.


 * Zerg: You can use broodling and guardians/lurkers can quickly kill groups of medics.


 * Protoss: You can use the dark archron ability feed back to kill medics you can also use reavers to target groups of medics.


 * All: Any units that can attack can kill a medic.

Ghost
A ghost can be a very big nuisance, what with their ability to cloak and call down nuclear strikes. The ghost also has the ability to use lock-down missiles, which can make them quite a pest to a mechanized army.
 * Terran: Missile Turrets work for defending against a nuclear strike. The Command Center's Comsat station works as a temporary solution. Only use vehicles or air units if they outnumber the ghosts.
 * Zerg: Use overlords to spot them in the battlefield and spore colonies to defend your base.
 * Protoss: Have observers around and a cannon or two in your base. Zealots, with their superior speed, can easily chase down a ghost, so long as there are observers overhead.

Vulture
The Vulture, when speed upgraded, is the fastest unit in the game. There are many strategies that can be used with them, including hit and run tactics. This unit can be upgraded with Vulture Mines, which will annihilate any group using starting units – Zealots, Zerglings, Marines/Firebats/Medics, and in some cases Hydralisks. The hit and run also works great in a small micro between Vultures and another unit.


 * Terran: Wraiths, Battlecruisers, and Goliaths will tear through Vultures. Watch out though, as Goliaths will be brought to 1 or 2 HP from mines.
 * Zerg: Guardians and Mutalisks will work the best. Lurkers and Hydralisks together will also take them down. Another tactic is to spawn a Broodling on one of them immediately after they lay mines, which will cause a mine group to attack the Broodlings. After that, mine splash will take care of the rest.
 * Protoss: Dragoons work for early game, and obviously any air unit. Do not use Archons, as hit and run attacks against Vultures are ineffective. However Archons float and don't trigger mines.

Siege Tank
Siege Tanks are definitely one of the strongest (if not the strongest) ground unit with its siege mode. The best way to overpower these tanks is:
 * 1) Quick, massive armies to surround the tank, as Siege Tanks can only attack far away units when in siege mode. By the time you surround it and the tank tries to change back to normal mode, the tank will be practically dead.
 * 2) Attacking with any air units. That simple.


 * Terran: Firebats and Marines accompanied by lots of medics – surround and kill them. Obviously, air units are also destructive against siege tanks. An army of Ghosts could wipe out an army of Siege Tanks as well. Make sure you upgrade the Lockdown for the Ghosts and use them against Siege Tanks to quickly wipe them out.
 * Zerg: Mass Zerglings and Hydralisks surround tanks. Attack the tanks one at a time unless you have a few dozen of them. Mutalisks can wipe out Siege Tanks easily and are fast, so if anti-air units come along they can retreat. Placing Lurkers right in front of tanks works well also.
 * Protoss: Same tactic as before, mass surround the tanks with any unit of your choice or attack with air unit.

Goliath
Think of the Goliath as a stronger and more specialized Marine. The Goliath's ground attack is not the most powerful, but has a good range, rate of fire, and deals 100% damage to all targets. Its anti-air missiles also do impressive damage. You should consider the Charon Boosters range upgrade for them to be mandatory. With it, they can match Gaurdians' range. One of the Goliath's downsides is its extremely poor pathfinding.


 * Terran: Marines are good at taking out unsupported Goliaths, as are Siege Tanks. Large numbers of Battlecruisers can work, but may take significant damage.
 * Zerg: Nothing tears through Goliaths quite like a Zergling group. Hydras can also work well, as they are more efficient against large ground units than Goliaths. Also consider Mutalisks, because Goliaths deal half damage to them, but not if the Goliaths have Marine support.
 * Protoss: Zealots with the speed upgrade and Dragoons with the range upgrade work well. Later on, use Reavers to deal massive damage to the Goliaths, or Dark Templars.

Wraith

 * Terran: Valkyrie, missile turrets, large groups of marines.
 * Zerg: Scourge, spore colonies, hydras with range upgrade.
 * Protoss: Photon cannons, scouts, dragoons with singularity charge.

Valkyrie
Valkyries are often very powerful units in mass, they have a high amount of armor, as well as hp,and it also deals splash damage. Valkyries can be quite a headache in the late game, when a strong air force is often essential to victory.


 * Terran: Battle Cruisers take a long time to batter down and can be very effective in numbers, to prevent them from getting wiped out, it is always smart to have scv's to repair your battle cruisers. If the enemy has no detectors laying around, wraiths can be very effective counters, given the strength of their air-to-air missiles. Any land unit with an anti-air attack will also work, seeing as Valkyries can't attack ground units.


 * Zerg: If you have some devourers on hand, because Valkyries are only effective in mass, they can spread the acid spores very quickly, which should nullify their armor. Hydralisks can easily finish the job.


 * Protoss: Valkyries are often thought to be effective against arbiters, simply because the splash damage can reach cloaked units. However, arbiters do have a long life-span against a typical Valkyrie fleet. If you are able to keep your air units outside of the halo missile's splash damage, yet still within the cloaking field, you will be able to mop up the mess with relative ease.

For All Races- Any anti-air base defense will leave a Valkyrie at mercy if the Valkyrie isn't backed up.

Science Vessel

 * Protoss : Dark Archon - use feedback its cheap has good range and 1 hit kills svs with alota energy. Just make sure the sv doesn't use an e.m.p.
 * Zerg: Scourge
 * Terran: Large groups of marines, or goliaths, can take out a science vessel very quickly. Wraiths also work, but cloaking doesn't, science vessels are detectors.

Battlecruiser
The Battlecruiser is an very expensive unit, but is arguably one of the Terran's strongest. It can serve many purposes, and is most effective in groups of six or more. These groups are extremely hard to stop. Also, the Battlecruiser can be upgraded to equip the Yamato Cannon, which can do 260 damage at the cost of 150 energy. Always accompany your Battlecruiser groups with Science Vessels, to provide defensive and offensive support including cloak detection.


 * Terran: You will be in for a fight either way. Try using packs of cloaked Wraiths against small Battlecruiser groups, or upgraded Goliaths. If you are facing a large group, counter with you own armada and target two Yamato Cannons on every one enemy Battlecruiser. If they have Science Vessels accompanying their Battlecruisers, bring Ghosts to Lockdown first the Vessels, then the Battlecruisers. If you do not have a group of Battlecruisers to match the enemy's, large numbers of Goliaths can do the job.
 * Zerg: No amount of Scourge will be able to hit large Battlecruiser groups (once upgraded, they one-shot kill Scourge), and Hydralisks are generally too weak to stand toe-to-toe. Use the Defiler's Plague to severely damage the Battlecruisers first, and bring in Devourers and Mutalisks to finish them. Hydralisks under Dark Swarm also makes Battlecruisers run.
 * Protoss: The Protoss have the best Battlecruiser counter: the Arbiter. A single Stasis Field can take 12 or more stacked Battlecruisers out of the picture, while you place Dragoons and Scouts around them to attack as soon as they are free. Also, large numbers of Photon Cannons can slow down a Battlecruiser group better than the other species' defenses. Or, use Mind Control on small groups. In lieu of these expensive solutions, upgraded Dragoons also work well in large enough numbers. You can also use the Dark Archons' Feedback ability, which will deal damage to Battlecruisers with remaining energy.