The way I always saw it was:
-Squirtle pays off the fastest because bubble makes beating Brock easier (beginning of the game)
-Charmander is most effective against Grass-type (middle of the game)
-Bulbasaur, etc. are most effective against ground (beginning and end of the game)

Bulbasaur is arguably the most useful starter, but for some reason seems to be the least popular. I never chose it, but I couldn't tell you why...

Do people have some unconscious bias against plant-dinosaurs?!