Warcraft III: The Frozen Throne/Units

The counter system of Warcraft III has been further refined in TFT. Air units in general counter melee, ranged and siege units, melee units in general counter ranged, caster and siege units, ranged units in general counter air units and caster units, and siege units in general counter caster units. Caster units aren't the counter to anything per se, but their abilities allow them to either support other units greatly, or to be the counter to something. For example priests are the counter to summons, and thus certain heroes.

Units, Buildings & Heroes
Spell Breaker They are one of the best counters to casters and heroes in the game. They are Melee-units which aren't exactly THAT strong, but their abilities are where they pwn: They can steal buffs from enemy units. That means if some orc plans to build bloodlust against you, build a few Spell Breakers and the bloodlust is yours. Their second ability is to burn mana with each attack. This ability burns 20 mana from all enemy units, and 4 mana from all enemy heroes. Additionally to that the amount of mana burned is dealt as damage to the enemy unit, which means that all units and heroes with mana die very fast when they are attacked by Spell Breakers. Their third ability is their spell immunity. This alone would qualify them as perfect anti casters, but they have one more ability: Control Magic. This ability has to be researched on Tier 3 in the Arcane Sanctum, thus making it rarely used, but should your enemy use summons, it will give you a huge advantage. This ability allows you to take over your enemies summons to your side. But the mana costs are huge - you will have to pay up to half the HP of the enemy unit as mana. (precisely it's 45%) But against certain summons with low hp (healing ward) this ability pwnz.

Counters: Human: Use footmen, use knights, or get the fragmentation shards upgrade for your mortar teams. Dragonhawk riders and towers can work too but will take forever to kill them. Night Elves: Use huntresses or druids of the claw. If you go with druids of the claw spam roar to avoid feedback damage and don't use rejuvenation in battle or the breakers will easily turn it against you. Undead: Ghouls and abominations both work wonders, I'd lean slightly towards the former thought just because of higher speed and cheaper cost. Orc: Grunts are good and tauren are totally own them. Wyverns can also be effective.

Dragonhawk Rider This is one of the strongest anti-air units in the game. That is because the dragonhawks ability to "shackle" enemy air units. This makes the enemy unit immovable for 40 seconds, and during that period deals a large amount of damage to the unit. The damage is enough to kill almost any air units in the game within the 40 seconds. However your Dragonhawk can not receive any new orders while he is shackling another unit, and the effect will stop if the dragonhawk is killed before the enemy is dead. (or if he is ensnared, or hexed, or stunned, or sleeped etc... The usual spells that stop a channeling spell) The other ability of the dragonhawk is great against enemy bases: Cloud. You have to research this ability first in your Gryphon Aviary. This ability allows you to disable all enemy towers within a large area. Two Dragonhawks are often enough to disable all burrows of an orc player. While this effect lasts the Dragonhawk can not receive any other orders, so you have to kill the buildings with other units.

Counters: Human: Flying machines and riflemen will obliterate them, your own dragonhawks can work too if you have superior numbers. Arcane towers will mean instant annihilation if they don't have cloud yet or their mana is too low to use it, but don't depend on them because all it takes is one cloud to make them useless. Night Elf: Use archers or enough hippogryphs to overwhlem them. Undead: Use crypt fiends with web or enough gargoyles to overwhelm them. Orcs: Use headhunters or raiders ensnare.

Blood Mage Very useful in 2on2 because of his flamestrike ability. His siphon mana ability allows him to drain mana from an enemy hero, or to give mana to a friendly hero. The banish ability makes an enemy invulnerable to physical attacks, yet more vulnerable to magical attacks. It is also slowed and can not attack. But it can cast spells. This ability is either useful to take out enemy units (sometimes you can banish all of his melee units, thus making them unable to block, and then easily kill his ranged units) or to make hero killing easier since spells deal extra damage, and you can surround him easier because he is slowed. His ultimate, the phoenix is a very strong summon.

Items
Scroll of Regeneration: This item is so very useful, that a shop should be built for it alone. With this item you can heal all of your units in a short amount of time. It can not be used in battle. Lesser clarity potion: The other item that you will buy all the time from your shop. It regenerates the mana of your heroes. Doesn't work in battle, too. Mechanical Critter: This item creates a critter for you which you can use to scout. It is... well... pretty much useless... You could in theory use it to position it next to a possible expansion of the enemy, but the sightradius is so small... Humans have got better possibilities for scouting. Potion of Healing: This item heals 250 hitpoints for a hero instantly. Useful in emergencies. Potion of Mana: This item heals 150 mana for a hero instantly. Sometimes useful. Ivory Tower: This item builds a scout tower anywhere you want almost instantly. This is really useful because you don't have to wait for the towers to finish. It doesn't even cost more than a normal tower. Town Portal: You should buy this in case you use the one you start with. Staff of Sanctuary: This item allows you to teleport a unit back to your home town instantly, where it has to stay until it is healed. Very useful in battles, because your units can fight to the very last moment, and you don't have to retreat them. However the cooldown is rather large, but you can pass the item around from hero to hero to get around this. And even though it's quite expensive it's almost always worth it. Orb of Fire: Adds +5 to the heroes damage, and makes his attacks do area of effect damage. Also allows him to attack air. You might consider buying this especially for the last effect.

Nightelf
Faerie Dragon

The Faerie Dragon is an anti-caster unit designed to counter spell casters, and can also work well against certain types of combat units. Its main ability, mana flare can be used to damage units that are casting spells, making it useful against spellcasters and heroes. Its spell immunity also makes it invulernable to negative spells and magic damage, making it hold up well against units dependent on those things like destroyers and sorceresses. Phase shift also allows it to dodge attacks, and coupled with its fast movement speed and the fact that it is flying makes faerie dragons very hard to kill. The only big disadvantages are that its damage is pretty low and against physical attacks and abilities that hit air units, faerie dragons don't hold up so well, so watch out for things like gargoyles, riflemen, and raiders with ensnare.

Undead
Destroyers

Destroyers are officially anti-caster units but they can be effective against many types of targets. Their orb of annihilation spell with it's magic attack boost and splash means total destruction against non spell immune units with heavy armor, light armor, or low HP like heavy air, melee, and workers. Their devour magic consumes buffs and summoned unit's health over an area of effect and turns both into mana and health for the destroyer, keeping them alive and allowing them to continue casting orb of annihilation, making them great against summon spells and buffs. Their spell immunity makes them immune to spells and magic damage, meaning anything with a magic attack can't touch them. And their absorb mana also takes a lot of mana from a friendly unit giving the orb plenty of energy. With these on your side nothing stands a chance...well except for the stuff listed below.

Counters:

Human: Use siege engines with barrage or guard towers with masonry upgrades. Flying machines are excellent if the destroyers don't have much mana for orb but otherwise they're screwed. Riflemen are also decent and can be more readily available than the others. Orc: Use watch towers, orc burrows, raiders's ensnare, bat rider's unstable concoction, berserkers, or headhunters. None of those are great but all are can work, and berserkers and towers are probably the best overall while headhunters and burrows are probably the weakest. Night Elf: Dryads totally own destroyers better than any unit in the game. Archers and hippogryphs are great at dishing out damage but not the best for taking it, while faerie dragons are the other way around. Ancient protectors in bases can help too.

Neutral Heroes & Mercenaries
Firelord:

At the beginning his spawns are very powerful and he is also a good choice for an Early Game Tower Rush as Human against Orc for example.