I'm thinking unicorn, because I really would like to be a mix of all three!
I could use the spells from the "Sonic Rainboom" episode to walk on clouds and have wings! That is, if I were as powerful as Twilight Sparkle.
Then, I could use spells to have the strength of an Earth pony! I WOULD BE THE MOST POWERFUL PONY IN ALL OF EQUESTRIA! *evil laugh*
It's all about magic, bro(ny).