I would choose Pegasus.
I mean how boring would it be to be an Earth Pony. Seeing all the flying the Pegasi can do, and the magic the unicorns can perform.
And how boring would it be to be a unicorn? Sure you have a piece of magical bone popping out of your head giving you magical powers. And sure, Twilight can cast a spell to give you butterfly wings. But those wings will not last like the Pegasus.
Pegasus is where it's at! You can fly, help with the weather, and go to Cloudsdale! I mean, do the Unicorns and the Earth Ponies have a place where they can only go? I don't think so!
So that my fellow Bronies, is why the Pegasi is the superior race. (Didn't mean to go all Hitler there, I love all the species, but Pegasus is my favorite.)