Chikorita, Bayleef, and Meganium
Chikorita is a pure Grass type Pokemon. As it evolves into Bayleef and Meganium, it will learn new Normal and Grass type moves as well as developing solid Defense and Special Defense. Along with decent offensive moves such as Razor Leaf and BOdy Slam, it will be able to learn valuable support moves like Synthesis, Reflect, Light Screen, and Poison Powder.
Despite sounding solid enough and being a decent enough Pokemon in its own right, it is not recommended for new players to pick Chikorita at the beginning of the game. This is because the Johto region is very hostile to a pure Grass type; of the eight gym leaders, six of their teams are resistant to Grass moves and/or are strong against Grass types. Although Chikorita would be strong against Water, Rock, and Ground Pokemon, these are relatively rare for most of the game.
If you want to use Meganium in competitive play, you'll find that it's not a bad option, though not top tier. It can either play a supporting role using moves like Reflect, Light Screen, and Synthesis to set up defenses for allies while healing itself, or if bred carefully, it can be a powerful Swords Dance user with Body Slam and Earthquake.
Cyndaquil, Quilava, and Typhlosion
Cyndaquil is a pure Fire type Pokemon. It and its evolved forms specialize in having high Special Attack and Speed. As they level up, they will learn a variety of Normal and Fire type moves.
Cyndaquil's main problem is that it takes a while for it to learn new Fire moves. By default, it will learn Ember at Level 12, Flame Wheel at Level 31 just before evolving into Typhlosion, and Flamethrower at the distant Level 60. You can speed this up by not evolving, but even Cyndaquil doesn't learn Flamethrower until Level 46.
Despite this slow acquisition of Fire moves, there's a lot to be said for Cyndaquil. The Fire type is fairly uncommon in Johto, especially before the mid-game, and can be invaluable for defeating the various Bug and Grass types early on and Steel and Ice types later in the game. The Fire type is also notably a decent match against all gyms except for the last in Johto.
If Typhlosion is used competitively, you'll find that it's one of the best Fire types available. Though it is vulnerable to Ground moves like Earthquake, it stands out from the likes of Charizard and Moltres by being capable of learning Thunder Punch. This gives it a way of fighting back against the various Water types that it outspeeds, as well as a way of dealing with the various Flying types used at the higher levels of competitive play. In addition to Thunder Punch, it should know either Fire Blast or Flamethrower and possibly something like Dynamic Punch or Earthquake for even better coverage.
Totodile, Croconaw, and Feraligatr
Totodile is a pure Water type. It and its evolved forms focus on having high physical Attack and Defense stats. They mostly learn Normal and Water moves by leveling up, though they also pick up Bite as a Croconaw.
Totodile is probably the simplest choice of a starter in Gold, Silver, and Crystal versions. It isn't as hard to use as Chikorita and doesn't suffer from a drought of Water moves similar to Cyndaquil. This is because by the time you'll want to replace Totodile's Water Gun, you'll have easy access to the HM for Surf.
That being said, there is a major downside to picking Totodile: the mismatch between its Water type and its physical stats. This means that Totodile, and later Feraligatr, will not be able to use Water moves as well as some other Water type Pokemon. The Water type is full of great Pokemon that you'll be able to catch, many of which have advantages over Feraligatr.
Competitively speaking, Feraligatr is probably the worst of the three fully evolved starter Pokemon. It is overshadowed by at least a dozen other Water types, such as Starmie, Vaporeon, and Suicune. If used, it should take advantage of both its Water type by having at least one powerful Water move, but should otherwise use physical moves like Earthquake, Rock Slide, Dynamic Punch, and maybe Curse to improve its stats. That being said, Feraligatr is probably not the right choice for whatever competitive role you want it to play.
Pidgey, Pidgeotto, and Pidgeot
Pidgey and its evolved forms are Normal/Flying Pokemon. This means that they compete with Hoothoot and to a lesser extent Spearow, seeing how the three are all Normal/Flying bird Pokemon found in the early game that grow up to become roughly comparable in power. So how does Pidgey stack up?
Pidgey's best advantages are its Speed 56/XX/91 and its early move variety. By Level 9, Pidgey will have both a Normal and Flying attack option in Tackle and Gust, as well as Sand Attack to hinder enemy accuracy. Having two STAB options early on will make Pidgey a fairly powerful fighter in the early game.
The problem with Pidgey is that even as it grows and evolves, its options don't get that much better. Sure, it will eventually replace Tackle and Gust with Quick Attack and Wing Attack, but these aren't the strongest moves. Even with TM's Pidgey and its evolutions will not get that many other options either. It also hurts that although Pidgeot technically has higher base stats than its competitors Noctowl and Fearow, they are distributed in such a way that Pidgeot lacks the Special Tankiness of Noctowl or the Attack and Speed of Fearow.
Pidgey may be worth using for a while in the early game, but it is recommended that you don't leave it in your party for the entire game.
Spearow and Fearow
Spearow is the last of the three Normal/Flying Pokemon encountered in the early game, only being found once you reach Route 33. However, it and its evolution Fearow eventually become the best of the trio.
Spearow starts with the move Peck, but otherwise it and Fearow don't learn many interesting moves for a while. It gets Fury Attack for a Normal type move, but how effective that is compared to Quick Attack is open for debate due to the move's variable nature.
What makes Fearow worth the investment are its high Attack and Speed and its access to Drill Peck. Drill Peck is arguably the best Flying type move in the game, being almost as powerful as Fly without taking a turn to set up. That being said, its moves are otherwise about as limited as Pidgeot's.
Hoothoot and Noctowl
Hoothoot is the third of the Normal/Flying type birds in the early game, and is one of the first new Johto Pokemon available to capture. Unfortunately, it is arguably the worst of the trio.
There are some upsides to using a Hoothoot or Noctowl. Along with having higher HP and Special stats than the other early birds, they have access to a few Psychic type moves. Hypnosis can be a valuable move in the early game, putting dangerous opponents to sleep. At higher levels, Confusion can be learned, as can Dream Eater.
The main problem with Noctowl is that it gives up too much of what makes Pidgeot and Fearow good with too little in return. Noctowl doesn't learn any Flying moves better than Peck, and although it does learn Take Down, this is only learned at the relatively high Level 33. Confusion is a useful move, but by the time it is learned at Level 41, other Pokemon will already have the stronger Psybeam or Psychic. And like its fellow birds, it doesn't learn much from TM's to compensate for its poor innate moveset.
Hoothoot is best used as a way to put enemy Pokemon to Sleep in the early game, either for fighting or to capture them. However, they are not a recommended permanent part of your party.
Rattata and Raticate
Rattata and Raticate are pure Normal types, the former of which evolves into the latter at Level 20. They are part of the "Com Mon" archetype of Pokemon designed to be used in the early game, but dumped later on in favor of stronger species. Despite this, they aren't a bad choice for a party member, especially in the early game.
Rattata can be found as early as Route 29, and continues to be a regular sight until your reach Goldenrod City. If you capture a Rattata, you won't necessarily be blown away by its overall staats, but you should note that it specializes in using its Attack and Speed to its advantage. Combined with the power offered by the Same Type Attack Bonus (aka "STAB") when using Normal moves, Rattata is a strong glass cannon in the early game, who learns new moves like Quick Attack and Hyper Fang at a reasonable rate. If you keep Rattata around after the first couple of gyms, it will evolve into a Raticate, which has stats on par with a second-stage starter Pokemon. One can then use it to cheese the Ecruteak City Gym by teaching it a move that hits Ghosts, since Ghost type moves can not actually harm Normal types.
With all this being said, Raticate loses its charm as the game grinds on, as other Pokemon start to outpace it both in stats and in having a variety of useful moves to potentially learn. Still, even if Rattata and Raticate are only a temporary addition to your team, they can still be a welcome one.
Raticate is generally considered to be very weak in competitive play, and is thus not recommended for use in that context.
Sentret and Furret
Pichu, Pikachu, and Raichu
The only way you're likely to get one of these Pokemon is if a Pichu hatches from the Odd Egg you receive on Route 34. At that point, you'll already have passed the point in the game where you can easily train a Level 5 Pokemon to become a main party member, especially, since Pichu requires a high level of Friendship to evolve. Considering how much unlikely it is that you'll even get a Pichu, how much effort it takes to raise it, and how uncommon the Thunder Stones necessary to evolve Pikachu into Raichu are, we can't recommend you use these Pokemon in your playthrough. Just get a Mareep instead, or teach one of your Pokemon Thunderpunch with TM41 from the Goldenrod Department Store.
In competitive play, Raichu is on the mid-to-lower end of the spectrum. It can use powerful Electric attacks, and if you can teach it a Water move like Hidden Power's Water form for coverage, you can give it a moveset that includes Rain Dance, allowing it to use 100% accurate Thunders instead of the weaker Thunderbolts. That said, this can be difficult to pull off if you're not transferring a Yellow version Pikachu with Surf to your copy of the game.
Caterpie, Metapod, and Butterfree
Weedle, Kakuna, and Beedrill
Ledyba and Ledian
Ledyba is a Bug/Flying type that evolves into Ledian at Level 18. Unfortunately, even compared to other Bug types available in the early game, Ledyba and Ledian are weak party members. Though they have impressive Special Defense, the rest of their stats are absolutely terrible. Ledyba's Attack is the 7th lowest in the game, and even Ledian's Attack stat is on par with Weedle's. This is catastrophic because the two only learn Normal type attacking moves by leveling up. Ledian doesn't learn its first decent attacking move until Swift at Level 42. The only saving grace is that if you somehow stick with Ledyba for long enough, you can buy it useful Ice Punch and Thunderpunch TM's from the Goldenrod Department Store, which use its slightly higher (though still bad) Special Attack.
In a game filled with lackluster Bug types, Ledyba and Ledian are solid contenders for being the worst of the bunch. They are not recommended for use in-game or in competitive play.
Spinarak and Ariados
Geodude, Graveler, and Golem
Zubat, Golbat, and Crobat
The Zubat line is a trio of Poison/Flying type Pokemon, with Zubat evolving aat Level 22 and Golbat evolving when leveling up with sufficently high friendship.
Despite Zubat being notorious as one of the worst species to use in the first Pokemon games, it is actually much improved here. Found on Routes 32 and 33 almost exclusively at Night, you'll capture a Zubat at Level 6 to 8, at which point it'll only know the less than great moves Leech Life and Supersonic. However, if you go through even a bit of effort trying to train one, you'll find that it gains good moves at a reasonable rate. These include the Dark type Bite at Level 12, Confuse Ray at Level 19, and Wing Attack at Level 27 or 30, depending on whether you evolve it or not.
Furthermore, if you travel around with your Golbat and it doesn't faint too often, you will likely have a fully evolved Crobat before Level 30. Crobat is a solid upgrade over Golbat, with all stats getting modest boosts except for Speed, which is massively increased.
None of this makes the Zubat line the best in the game. But taken on its own terms, it is a very powerful counter to Grass types and a good way to beat the various upcoming Ghost types in the game.
In competitive play, Crobat is considered to be moderately useful, with amazing Speed but relatively few useful moves to use with it. Still, Crobats with moves like Wing Attack, Toxic, Confuse Ray, Screech, and Hidden Power are useful either on the offensive or spreading Poison to enemy teams.
Cleffa, Clefairy, and Clefable
Igglybuff, Jigglypuff, and Wigglytuff
Togepi and Togetic
Sandshrew and Sandslash
Ekans and Arbok
Mareep, Flaaffy, and Ampharos
Mareep can be found on Route 32 fairly early in the game, along with being available much later on at Routes 42 and 43. The Mareep family is purely Electric type, and evolves at Levels 15 and 30.
Despite players likely not encountering Mareep until after they clear Violet City Gym, it could still be wworth going out of your way to catch a Mareep. Found at Level 6 on Route 32, they can quickly be taught Thundershock by Level 9, at which point they become very effective at fighting most of the fairly common Flying and Water Pokemon you'll encounter. Furthermore, although Mareep's stats are a touch underwhelming, its Special Attacks is good for the early game, and it receives significant power spikes fairly quickly from evolving as early as Level 15.
As it evolves, Mareep will learn only a few notable moves after Thundershock. Assuming you let it evolve into a Flaaffy and Ampharos, it will learn Thunder Wave at Level 18, Thunderpunch at Level 30, and the mighty Thunder at Level 57. That is plenty of Electric type coverage, but not much else. Thankfully, the Mareep line, especially Ampharos, can learn some very useful TM's. If you get a Mareep, it is worth picking up TM48 at the Goldenrod Department Store so you can teach your Flaaffy/Ampharos Fire Punch ASAP.
Despite being a solid choice in singleplayer, Ampharos is overshadowed in competitive play by even more powerful Electric Pokemon such as Zapdos and Raikou. However, it has a place in the middle tiers as a reasonably powerful and bulky Electric type.
Wooper and Quagsire
Gastly, Haunter, and Gengar
Do not attempt to use Unown in your party.
Unown is something of a novelty Pokemon, whose gimmick is that there are different forms of Unown for every letter of the Latin alphabet. The game also gives the player a quest to hunt down every variety of Unown for the Unown Mode of the Pokedex. However, there is no in-game reward for completing this part of the Pokedex.
In battle, Unown is a weak Psychic Pokemon that only learns a single move, Hidden Power's gimmick is that it is a different type for different individual Pokemon, meaning it can be taught to a Pokemon to improve their moveset's type coverage once you get the TM. However, as the sole move of a weak Pokemon, it is a poor move.
Onix and Steelix
Bellsprout, Weepinbell, and Victreebell
Hoppip, Skiploom, and Jumpluff
Paras and Parasect
Poliwag, Poliwhirl, Poliwrath, and Politoed
Magikarp and Gyarados
Goldeen and Seaking
Slowpoke, Slowbro, and Slowking
Oddish, Gloom, Vileplume, and Bellossom
Drowzee and Hypno
Abra, Kadabra, and Alakazam
Ditto is a Normal type Pokemon with one move: Transform, which allows it to copy the moves and stats of opponents. However, it is generally weak in battles, given that it essentially turns any fight into a mirror match where the opponent gets a free hit in.
Fortunately, Ditto has another use that makes it valuable to players: its breeding capabilities. Any Pokemon that's capable of breeding, regardless of gender, can breed with Ditto. This makes breeding Pokemon much easier once you get a Ditto, since you only need to capture one member of a species rather than two.