My opinion:
Grass: Sceptile - with a overall stat of 530, a high sp.attack and speed but a low defense.
Fire: Charizard - with a overall stat of 534, a high sp.attack and speed, and a good set of moves.
Water: Swampert - with a overall stat of 535, a high hp, attack and a good balance of def and sp.def, and a good variety of move-sets.