I think Torchic is the best. Once you get hi jump kick from the tutor, Blaziken is a wrecking ball. A couple of Bulk ups and it sweeps pretty much anything. I beat a lot of gym/boss battles this way that i actually felt bad. Froakie is also good, but not at the same level imo. Lack of good TMs hurts. I'm surprised noone mentioned treecko. It's a staple powerhouse. Like someone else already said, i wouldn't pick any starter that can be obtained in game later on. This includes Fennekin, Squirtle, Chikorita, Cyndaquil, Mudkip, Bulbasaur, Chespin, Tepig. (probably forgot someone, but you get the point). Sure Fennekin and Mudkip sweep the early gyms, but in the long run having more pokemon to choose matters more than how easily you got to late game.