Chikorita is available as the Grass-type starter in the Johto-based games.
In the earlier generations of Pokemon, the starters usually fall into one of three categories: "best," "okay," and "bad." Unfortunately for Chikorita, it falls into the third category. Many players would even call Chikorita the worst starter of all time. It's not that Chikorita is objectively a "terrible" Pokemon; it's just that the Johto region is just extremely unkind to our leafy friend. Sporting a mono-Grass type, the Chikorita line technically has a type weakness to an astonishing SIX out of the eight Johto gyms - and that's not even including the many, many Poison, Flying and Fire-types possessed by Team Rocket. To add insult to injury, Meganium suffers mightily against several key members of the Elite Four, such as Koga's Crobat, Karen's Houndoom, and Lance's Dragonites. Finally, while both the Cyndaquil and Totodile lines gain access to a plethora of powerful attacking moves, both STAB and coverage, Meganium is essentially limited to the rather weak Razor Leaf, Body Slam, and Earthquake.
There is, however, a silver lining. Although Chikorita is certainly lacking on the offensive front, it has great natural bulk as well as solid support options such as Synthesis, PoisonPowder, and dual screens (unfortunately no Leech Seed or Sleep Powder though). Yes, Chikorita's Grass-typing is subpar at best, but when combined with its bulk, it provides the best matchup out of the three starters against rather threatening 'mons like Whitney's Miltank, Chuck's Poliwrath, and Clair's Kingdra. The fact that it matches up very well with these mons is basically the only reason to use Chikorita over the other two starters. If you are looking for an extremely difficult but very rewarding challenge, then choose Chikorita.
Rival (Cherrygrove City): Be at level 7 or higher and spam Tackle, or use Razor Leaf and hope for crits. Simple battle.
Gym #1 - Falkner (Violet City, Flying-type): You can set up a Reflect and then potentially stall out with PoisonPowder, but this will only work if you are several levels higher. Alternatively: use Razor Leaf and hope for crits. You must maintain Reflect, or Chikorita will surely perish. Being overleveled helps, too - a level 14 Chikorita can survive the battle without healing.
Gym #2 - Bugsy (Azalea Town, Bug-type): It may sound like a good idea on paper to try to use the aforementioned Reflect + PoisonPowder combo on the Scyther, but in reality, it will only temporary slow down the impending doom of a Fury Cutter sweep. The cocoons are mostly harmless, though. Feel free to take them on if you want, just watch out for Kakuna's Poison Sting possibly poisoning Bayleef.
Rival (Azalea Town): Chikorita has the possibility of losing to each of these mons one-on-one. If you absolutely must fight one of them, the Gastly is the least harmful of the three, though you will not be able to touch it with any of your moves, so you can set up a support Reflect and switch out on it.
Gym #3 - Whitney (Goldenrod City, Normal-type): Get off a PoisonPowder ASAP and pray to Arceus that hax is in your favor. If you're afraid that the Miltank will sweep you clean, then she probably will. It's a good idea to be slightly overleveled for this battle.
Rival (Burned Tower): Bayleef matches up decently well against Magnemite, especially if it is packing Mud Slap. (Keep in mind, however, that a super-effective Mud Slap counts as less BP as a not very effective Razor Leaf, but Mud Slap is still the better option because of the accuracy drop.) Same goes for the Gastly. It can also defeat the Zubat with Headbutt. Definitely avoid the Quilava though.
Gym #4 - Morty (Ecruteak City, Ghost-type): Bayleef does decently well if it uses Mud Slap; however, Spite will ruin your life due to Mud Slap's mediocre 10 PP. Definitely carry a Mint Berry for the Gengar.
Eusine (Cianwood City, Crystal only): Bayleef fares pretty decently in this fight. PoisonPowder-stalling Drowzee is the way to go. Keep in mind that Strength or even Headbutt in this case will do more damage to Drowzee that Razor Leaf will due to Drowzee's high Special Defense stat. It's best to avoid Haunter. Bayleef matches up extremely well against Electrode as it can tank SonicBooms and Thunders all day long.
Gym #5 - Chuck (Cianwood City, Fighting-type): Bayleef can actually solo this fight provided you play your cards right and you don't fall prey to Hypnosis/DynamicPunch hax. Meganium will win this fight easily, with both offensive and defensive power.
Gym #6 - Jasmine (Olivine City, Steel-type): You should actually be able to win this fight without any major problems, although it will probably take a while. All your moves will be resisted by Jasmine's Pokémon, so it's definitely not advisable to use Meganium as your main 'mon here, but it can tank attacks and dish them out relatively well in this battle. It's also useful for an emergency healing of another Pokémon.
Rocket Executive battle #1 (Team Rocket HQ): Meganium definitely has its work cut out for it. You will not be able to fight most of these battles unless there is no clear type disadvantage. Raticate is one such case, but Koffing and Zubat are not.
Rocket Executive battle #2 (Team Rocket HQ): Not much better as a matchup, since Arbok can only hit super effectively with Poison Sting, which is not really scary; however, Gloom has Acid, and Murkrow has Pursuit and Wing Attack. Bail out of this as soon as possible.
Gym #7 - Pryce (Mahogany Town, Ice-type): You can actually win this fight because Meganium is immensely bulky and has STAB super-effective Razor Leaf on each of his three 'mons. Icy Wind will slow you down, however -- both metaphorically and literally. Try to avoid having Meganium solo this fight. Piloswine also has Blizzard, which you do not want your Meganium to have to take. However, Razor Leaf should be able to put it down in one hit. You should be faster than Piloswine at this point in the game, but check before this battle to be sure.
Rocket Executive battle #3 (Goldenrod Radio Tower): Again, it is best to let a teammate handle the bulk of this fight, if not all. Everything can explode and takes little damage from whatever Meganium may have.
Rival (Goldenrod Underground): Meganium can potentially overpower the Sneasel, Magnemite and Haunter just by spamming Razor Leaf, as none of those three mons can deal any significant damage to our leafy friend. If you really want to go up against the Quilava, it is entirely possible if you have Mud Slap, but not advisable, as you don't want your starter's life to depend on accuracy hax. Definitely avoid the Golbat (his lead) as it has STAB super-effective Wing Attack.
Rocket Executive battle #4 (Goldenrod Radio Tower): A repeat of the HQ fight with an evolved Vileplume, whose Acid is scarier than ever. Arbok is still kind of alright, but the other two are as troublesome as they were before and Meganium is not cut out to fight them.
Rocket Executive battle #5 (Goldenrod Radio Tower): Fire, Poison, Fire. No good. Stay away.
Suicune (Bell Tower, Crystal only): Meganium absorbs Suicune's hits very well, including Gust. Unfortunately, its Rain Dance will make Synthesis recovery harder, but Razor Leaf alone should do the trick and you can put up Reflect and/or Light Screen (if you already have it) to reduce the damage.
Gym #8 - Clair (Blackthorn City, Dragon-type): Meganium can actually hold its own in this gym, being able to take out the Kingdra with relative ease as long as you are not extremely underleveled.
Rival (Victory Road): You can handle his lead, Sneasel, but just make sure to kill it ASAP because it may try to spam Fury Cutter. Both Magneton and Haunter still can't do a whole lot of damage to you, so you can outstall them. If not outstall them, you can set up screens on them and leave the killing to other, less bulky teammates. Definitely avoid the Golbat and Typhlosion, of course.
Elite Four Will (Indigo Plateau, Psychic-type): Meganium can easily beat the Slowbro one-on-one. You can tank a few Psychics from Exeggutor and the two Xatus and in turn Poison stall them, but this will take a while and leave you prone to crits/SpDef drops. Avoid the Jynx altogether as it will easily 2HKO you at worst with STAB Ice Punch.
Elite Four Koga (Indigo Plateau, Poison-type): This will be a very tough matchup, even if you have Earthquake and Body Slam. The Ariados can't touch you, but can just spam Double Team and then Baton Pass away. Venomoth will just Toxic stall you to death, although you can try to KO it with Earthquake. Forretress will set up Spikes and then eventually Explode on you if you don't KO it in time, which you won't. You may be tempted to use Earthquake on Muk, but please don't, since it will definitely not OHKO and Muk may OHKO with Sludge Bomb. Crobat will outspeed and probably 2HKO you with Wing Attack.
Elite Four Bruno (Indigo Plateau, Fighting-type): Meganium can set up a Reflect and stall everyone out, but you run the risk of a crit Cross Chop from the super powerful Machamp. The three Hitmons and Onix are really easy to KO, however.
Elite Four Karen (Indigo Plateau, Dark-type): You can try to poison-stall Umbreon, but just be wary of the possibility of Karen using Full Restore on it right before it KOs. Avoid the Vileplume as it has Acid and Moonlight. Murkrow doesn't have any Flying attacks, so you're good to poison-stall there as well. Gengar can be KO'd with Earthquake, but you are testing fate as it can use Destiny Bond and should outspeed. The Houndoom will surely burn you alive with Flamethrower.
Champion Lance (Indigo Plateau, Flying-type): The only way Gyarados can deal any significant damage to you is through Hyper Beam, so you can play around that. One of the level 47 Dragonites has Thunder and the other has Blizzard, so if you manage to figure out which is which, then you're golden. The level 50 Dragonite has Fire Blast, so unless you feel ballsy enough to try PP Stalling, you should probably let someone else handle this matchup. The same goes for the Aerodactyl and Charizard, both of whom have STAB supereffective moves.
From this point onwards, you can fight the gyms in any order, though you will need to retrieve the Machine Parts from the Cerulean City gym before you have access to the earlier portion of Kanto. Feel free to anticipate or postpone any battles as needed.
Gym #9 - Brock (Pewter City, Rock-type): LOL Razor Leaf. Everything is x4 weak to it.
Rival (Mt. Moon, optional): Setting aside the usual Golbat and Typhlosion, Earthquake can handle Gengar well, but Destiny Bond does exist and you should probably not try your luck. Magneton falls instantly to it, though. Alakazam hits hard, but Meganium's bulk is also very good and you can use Light Screen to absorb more damage while you whittle it down. As for Sneasel, as usual, take it out fast lest it spams Fury Cutter.
Gym #10 - Misty (Cerulean City, Water-type): Meganium's Razor Leaf alone is a great asset in this fight. Be wary of Starmie and Lapras though, as they have Ice coverage. Put up Light Screen before they come into battle and you should be fine, but critical hits will still get you and, depending on your level, potentially kill you.
Gym #11 - Lt. Surge (Vermilion City, Electric-type): A Meganium with Earthquake fears nothing in this gym, including the Electrode's Explosion. However, bring X Accuracy just in case, because they will still probably move first and they love spamming Double Team. Once you hit them, they're gone.
Gym #12 - Erika (Celadon City, Grass-type): This matchup isn't ideal. You can poison Tangela, Jumpluff and Bellossom, but they take little to no damage from your other moves. Victreebel has Acid too, so avoid that.
Gym #13 - Janine (Fuchsia City, Poison-type): Since everything here is at absurdly low levels, Earthquake should do the trick for all but Crobat. However, use Meganium only if you have no other options for the Weezing, they still have Sludge Bomb. Venomoth doesn't have Poison moves besides Toxic and Ariados doesn't have Poison moves AT ALL, so go ahead.
Gym #14 - Sabrina (Saffron City, Psychic-type): You can absorb hits fairly well by putting up Light Screen. Then, fight with Razor Leaf if they have Reflect up, else rely on Earthquake or another physical move. Watch out for Alakazam's Future Sight; keep count of the turns and don't let too much damage stack up. Otherwise, Meganium is fine here.
Gym #15 - Blaine (Seafoam Islands, Fire-type): Probably not, except you can Earthquake the Magcargo which is easy to defeat anyway. Leave Magmar and Rapidash to someone else, though, because even Light Screen won't save Meganium from super effective Fire hits under Sunny Day.
Gym #16 - Blue (Viridian City): You can go laugh at Rhydon and then stall Alakazam and Exeggutor with the usual techniques. Arcanine is best avoided, for obvious reasons; Gyarados is a mixed bag: you can hope to take it out, but Hyper Beam hurts a lot, so tread carefully and keep Reflect up if you must. Similar considerations hold for Pidgeot: it "only" has Wing Attack, but it's annoying to take down.
Rival (Indigo Plateau, optional): The fight is pretty much as usual, except Golbat evolved into Crobat. Predictably, it's no more viable than it was before. Avoid Typhlosion too, possibly also Gengar, and take out the rest.
Red (Mt. Silver): If you haven't already done so, you absolutely want to go to Celadon, go into that little food market place, look into the trash, and get the LEFTOVERS. GSC are the only games that have Leftovers in this location. Anyways, with the right moves (dual Screens, PoisonPowder, Body Slam, Razor Leaf, and possibly Earthquake), Meganium actually has a decent matchup against 5 out of the 6 mons that Red has. Pikachu is easily defeated with Earthquake. You can stall out Espeon's Psychics, but they will hit like a truck and you really don't want that SpDef drop. Snorlax is a no-game since it will eventually Body Slam you to death and you can't do squat back to it. Venusaur does not have any Poison-type attacks, so you can stall it out. Blastoise does have Blizzard, but you should still win if you allow yourself to use items liberally; non-STAB Blizzard probably won't be able to OHKO, and since Blizzard has low accuracy and low PP, you should be able to stall it out of the 5 PP that it has.
Chikorita's movepool is definitely the most barren out of the three starters. It gets access to a 55 BP STAB move in Razor Leaf at level 7, which sounds fantastic on paper, but it not all that good when you consider its many resists early-game. The Chick also learns Synthesis at level 22, which can be very useful for a limited-item run. PoisonPowder is very useful as well, allowing The Chick to outstall a variety of foes that it probably could not beat otherwise. Along the same lines, Reflect and Light Screen (which Chikorita learns at level 12 and 36, respectively) are extremely important stalling moves, but they give Chikorita a great deal of Four Moveslots Syndrome. Body Slam, which Bayleef learns at level 32, is a very solid attack with a 30% paralysis chance.
Finally, the only TMs worth mentioning are Headbutt, which has a nice flinch chance; Earthquake, which allows Meganium to potentially beat some Steel, Fire and Poison-type foes; and Toxic and Giga Drain, which sadly enough you do not receive until after arriving in Kanto. Remember that TMs in this generation are one-use, and decide carefully whether it is worth investing into Meganium as an offense-move user over another teammate.
Recommended moveset: Razor Leaf, Earthquake / Body Slam, Body Slam / Reflect / Light Screen, Reflect / Light Screen / PoisonPowder
Minimum stats are calculated with 0 EVs, IVs of 0, and a hindering nature, if applicable.
Maximum stats are calculated with 252 EVs, IVs of 31, and a helpful nature, if applicable.
Chikorita Line Ratings
At what point in the game should I be evolved? As with virtually every single GSC first-stage Pokemon, you absolutely have to have a Bayleef by the third gym (Whitney). The Chikorita line's success in Gyms 5-7 is widely dependent upon whether or not you have managed to evolve it into a Meganium, so you probably want to put off the fifth gym for as long as possible (as it is by far deadlier than both the other gyms combined.)
How good is the Chikorita line in a Nuzlocke? Compared to the other starters, Chikorita is absolutely terrible. But compared to other available Grass-types, Chikorita is probably the best (although that's not saying much). Granted, Meganium definitely has the best matchup against Red, but it is doubtful you will survive that long. Unless you really want to challenge yourself, you're better off picking one of the other two starters.
Weaknesses: Fire, Poison, Bug, Ice, Flying
Resistances: Grass, Water, Electric, Ground
Neutralities: Dragon, Dark, Psychic, Fighting, Ghost, Rock, Normal, Steel