Bulbasaur, it learns sleep powder at level 15, making it good for catching other pokemon, it easily beats the fisrt two gyms, is resistant to the third and forth and also has an easy time with the last.
Charmander is...meh, but can be taken down in one rockslide, and Squirtle is more useful with mirror coat, which requries breeding.
Overall it doesn't matter which you choose, ingame the are relatively all as usefull as each other.
But saying that, Bulbasaur is proberly the most useful for filling up the dex /catching more team members(sleep powder), as well as allowing you to easily beat the first four gyms. So i'd pick Bulbasaur.