Personally, I would prefer being able to choose between SIX starters. The Grass/Fire/Water starters are old and lame. DP missed a great opportunity to take the game in a new direction.
As for the best Pokemon in the game, that would be Snorlax. Charizard, being a Fire-type automatically loses as the best starter. That title goes to Swampert. It has the best typing (being a Water immune to Electric attacks is HUGE), has only ONE weakness, learns a great move to cover that weakness, has a great movepool, and can be used as a tank, a physical sweeper, a special sweeper, or a mixed sweeper effectively. It also learns three moves that takes down Charizard - Surf, Rock Slide, Ice Beam. No comparison.