Unicorn. Here are my reasons:
Magic
Wing-creation Magic
Efficiency (think about Twilight's magic at the end of Applebuck Season)
Nobility
If you had to choose between Appleloosa (Earth ponies), Cloudsdale (Pegasi), or Canterlot (Unicorns), which would you choose?
Okay, bad question. But Cloudsdale reeks of capitalism and Appleloosa's not for me.
Jobs (as a class)
Notably, the Earth ponies are known for their agriculture, and the Pegasi are mostly serviceponies when it boils down to it (weather factories).
Lack of limits
I don't know so much about this one, but magic seems a lot more mysterious than flying and athleticism.
Did I overanalyze again?