Honestly, they're all kind of cool in their own way, so I can't pick?
Earth ponies have something of the "everymare" or "underdog" effect, because they don't seem to have as cool or flashy powers. We humans don't either, so it makes them relatable and sympathetic - especially since we don't have to live together with more magically awesome races. And yet many earthponies are awesome too, in their own ways, and as a people they feed all of Equestria. It would have been nice if their tribal magic had been explored more instead of just hinted at, but subtle powers can still be important. And they seem to have some of the strongest famillial and communal bonds.
Pegasi tap into an age-old dream, soaring freely in the sky (well, except Scootaloo) and exerting control over the weather we mere mortals have to just put up with. Their world among the clouds is probably the most fantastical part of Equestria, a realm for only them in the way the other tribes don't have. As a one-time warrior society, they retain a mix of bold passion and a strong sense of duty, embodied in a tradition of public service. The boldest mavericks and the most steadfast officials of Equestria likely spring from this tribe, along with as many other kinds of pony as there are kinds of weather. And being the "middle" tribe and dwelling at something of a remove, they can be somewhat overlooked...
Unicorns' magic is very convenient - telekinesis would be so alone, but each unicorn also has access to magic based on their talent (even if we don't see this much) and that's before getting into great mages. But unicorns aren't just about magic - there's also art, fashion, music, scholarship and all other kinds of high culture. Not that all unicorns are rich or snobby, but although they walk on the ground like earthponies their heads are often lifted to the stars. I can see them as the origin of Equestria's greatest dreamers and visionaries, as well as it's most hidebound conservatives.
Crystal ponies have been criminally overlooked - but on the other hand, this lets us form our own ideas around the little that we know. (I go for a society based heavily around empathic magic and positive emotions.) And of course, whatever culture they once had, there's also the "recent" tyrannical regime and thousand-year timeskip to add extra baggage.
Bat ponies... probably don't exist. But if they did, they'd be pretty cool - natural dreamwalkers, perhaps? Same for real seaponies (sorry, hippogriffs, but ripoffs don't count).
And last but certainly not least, alicorns. The living embodiments of the unity of the three tribes. Some prefer them to be rare and exclusive. Others see an all-alicorn Equestria as a goal to be aspired towards - although non-ponies tend to find that idea terrifying instead. They can spawn countless arguments, from their origins to their (im)mortality, but even if you don't like some of them, I think we can mainly agree the world of FIM is richer for having them.