I would be an earth pony.

Earth - Earth ponies are more in touch with nature and are much stronger then pegasi or unicorns, as well as, sometimes, larger.

Pegasus - Pegasi can fly and can also control the clouds. Most other ponies, unless assisted by magic, would fall through a cloud like it's vapor. They're the fastest of the three.

Unicorn - Unicorns can tap into magic with their horn, though for most unicorns the magic they can use is limited to their talent. They're the smartest.

Alicorn - Has all the powers of the other three. Well-built and agronomic, flying and quick, as well as (extremely) magical and smart. They're also immortal... pretty much pony Goddesses. They're usually a horrible self-insert. If you choose this, you're likely a self-inflated douche.

Which kind of pony would you be? You can either choose your preference or base your decision on your character. Or both. Whatever.