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Diamond=Chimchar (only starter i've had on it)
FireRed= Charmander (i've had a bulbasaur and it's good to start with but i feel charmander is better in the long run as i evolves into charizard and he can get pretty powerful.)
Infernape is the most widey used in competeitive play, I think, though I believe that Swampert is as well and occasionally Empoleon. That might not be quite right, though...