Bulbasaur's strongest move he has for a long time is just razor leaf and he has a crapload of weaknesses. I dont need to explain chikorita Treeko has to rely on absord/mega drain until he gets leaf blade at lvl 29 Turtwig and grotile are pathetic Torterra is decent if you can live with being outspeeded by everything and its grandma Snivy is just awful in-game Chespin is admitingly good.
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Bulbasaur is really solid. Beating Misty is good, and Sleep Powder capturing and battling a lot easier. When compared to Squirtle and Charmander, I'd say it's on-par with Charmander (Who has trouble with Misty but is better offensively) and behind Squirtle (Who shines through the entire game because Water+Blizzard=win).
Treeko is the worst of the Hoenn starters, but that's not saying much because all three are great (With Mudkip being the best). It has a poor movepool at first, but then it gets some decent attacks and just wrecks most of the Water Pokemon in the later game with its high Special Attack and Speed. Sure, it can't beat Wingull and Tentacool, but you don't actually fight wild Pokemon with something that fast and trainers have way more varied teams. It's best in Emerald where it just destroys Wallace when combined with any Pokemon with a decent Grass attack.
Turtwig is slow, but it has decent power and a nice level-up moveset. Once it reaches Torterra status, it becomes a lot nicer because URFQUAKE beats in-game battles handily. It's completely outclassed by the other two starters, but it's not bad by any means (And Piplup's completely outclassed by Chimchar anyway).
Snivy is pretty awful in-game until you start reaching longer battles where Coil->Sweep begins to shine. Still, that movepool is dreadful outside of Coil and Leech Seed. It's sad, because Serperior could have been really solid if its movepool was anywhere near as good as Chikorita's.
Speaking of Chikorita, its problem surprisingly isn't that it sucks or that its movepool sucks; its problem is that Johto screws it. Terrible matchups against the gyms, terrible matchups against Team Rocket, terrible matchups against the Elite 4. Being a defensive Pokemon would be bad enough, but its real issue is that it just doesn't get any advantageous matchups. If there were Water, Rock, and Ground gyms in Johto instead of Kanto and we were fighting Team Aqua instead of Team Rocket, maybe Chikorita wouldn't suck in-game.
Chespin... Well, it had the makings of a great starter. Its main weakness is that it doesn't learn any decent Fighting attacks for most of its life. It might be better than Froakie because it can take a hit and actually deal damage, but Fennekin is clearly the best starter in-game.