Rainbow Dash, so she could teach me not to be a wuss!! lol
Joking aside, I'm a bit of mix between Twilight and Fluttershy personality-wise (Caring, shy, a complete nerd, and a bookworm) so I think an athletic and courageous pegasus like Dash would be a complement to my personality, because most of my friends in real life are like Pinky Pie or Dash. Fun and outgoing, but very competitive.
Anywho, she'd rip me out of my shell, and then we'd have epic races against each other.