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Hard choice, OP. Personally, I love bulbasaur, and by extension, Mega Venusaur, but don't care for the other two.
That being said, Bulbsaur's probably not your best bet, just because it's not new.
Even though I don't care for the other two, Gogoat's looking solid as hell. Good attacking stat along with a self recovery move, and amazing SpDef (based off of it's ability to shrug off an ice beam from Slowbro).
Chespin, if it does indeed evolve into Grass/Dark, should also be weighed against any other possible Dark types you might want to use.