RBE: Mudkip. Nothing is more comforting than reaching the late game with a starter that can learn Earthquake, Surf, and Ice Beam. Many, many more uses than the others.
FRLG: Bulbasaur. People say it's only good for the first couple of gyms and it sucks against the last few, but I disagree. By the time you get to them, you have the resources available to deal with them via other means.