Bulbasaur > Ivysaur > Venasaur:
Grass/Poison. 4 weaknesses and 4 resistances. ok.
Decent Sp. Atk, Sp. Def, and Speed. good, but normal Atk and Def are lacking.
Chikorita > Bayleef > Meganium:
Grass. 5 weaknesses and 4 resistances. hmm.
Good Defense and Sp. Def and decent speed. I myself prefer good all-around defense if speed is below 100 base stat for final form.
Treecko > Grovyle > Sceptile
Grass. 5 weaknesses and 4 resistances. hmm.
Excellent Speed and Sp. Atk. normal defenses are lacking. but if it is fast enough with a high enough Attack or Sp. Atk, it might not be such a problem.
Turtwig > Grotle > Torterra
Grass > Grass/Ground. 5 weaknesses, 4 resistances to 4 weaknesses, 2 resistances, and 1 immunity. hmm...
Decent Attack and Defense, but REALLY BAD SPEED. Its Sp. Def is ok, but its Sp. Atk is not that good so there isn't much use for its special attacks.
i go with...Treecko