Well, I don't really like any of them that much. There should been an option for Spyro or Samus. I would've choosen one of those two. But I was forced to choose Peach because of the random asortment of characters.
I say Mario because he's like the king of platforming games and he never fails to amaze me. I loved Mario, getting mushrooms, shooting fireballs, destroyinh bricks, and so on.
But then I discovered Sonic, which was pretty much as fun. But since I never got the hang of Sonic and I'm such a n00b in games, I prefer Mario over Sonic.