Chimchar, because Grass and Water is easy to come by, yet Fire is really rare, especially early on.
Also, the Fire Starter is one of the Only 3-stage pokemon in the entire pokedex, and I really like to have Pokemon that evolve twice as my fighters.
So chimchar.
EDIT: I also have an Empoleon. GOD, it's annoying. Earth types can kill it easily, and it's not defensive against Fire.
I don't really like Grass types because they don't gave many good solid attacks. I.e. Whilst you can get Flamethrower and Fire Blast for fire, Grass doesn't really have those types of moves.