Bulbasaur. Why? Probably the best grass type in the game. Good against the first three gym leaders, along with Geovanni. Also a bamf for the many dungeons in the game.
I know Charmander was always more popular, but I never chose one more than once or twice. Why?
1. Fire types are weak (Good against...grass? bug? ice? Who uses bug? And there are only two ice pokemon in the entire game who aren't also water type. They're also weak against ground, rock, and water types, which are probably the most commonly encountered types in Red and Blue.*)
2. There were better fire pokemon (Arcanine, Ninetails)
3. Charizard picks up flying, which ruins him because it just adds another flyer to your roster that I can take down in one hit with Jolteon...
*EDIT: Besides normal and flying types, that is...