for me it's bulbasaur, but i guess most people disagree. but really, grass is strong against rock, water, electric.. and no significant weakness against grass... bulbasaur is definitely the best starter in terms of beating the game because he's strong against most of the gym leaders(the early ones at least) whereas squirtle would be weak against electricity, and charmander weak against well, all the early gym leaders until grass.
in terms of their actual strength, i'd still say bulbasaur. leechseed+sleep powder is a deadly combination in competitive battle with friends and against gym leaders, well, they're all strong. bulbasaur does have his weaknesses(against physic, fire, especially) but really, every pokemon has his weakness and i think his strengths far outweigh his weaknesses.