I would be either Riolu, Grovyle, or Charizard.
Riolu because he's small and cute, but can pack a punch if used right.
Grovyle because I'm not very athletic, but I think it would be cool to be fast and agile. Plus a built in blade? Yes please. :D
Charizard because I'm short, and always wonder what it would be like to be taller (Charizard is taller than me) and I wouldn't need to pollute the environment with a car because I could just fly anywhere I needed to go.