Everyone who's not named Lightning Dust is a less accomplished and skilled flyer than Rainbow to be fair.
The Wonderbolts are apparently the only flying team in all Equestria that's not just a city-based team raised only for the Equestria Games from what the show gave us too. Might be a case of there being literally no one else for her to join.
But they're also some kind of military who get sent to deal with monsters as seen in Secret of My Excess. So that's a job that'd let Rainbow be the best flyer around, get into awesome fights (see her dream in Do Princesses Dream of Magic Sheep), help ponies and look amazing doing that the whole time.
So yeah. Even if there was a better flying team around, the Wonderbolts are Rainbow's dream team. They're kinda similar to her idea of Flash Magnus come to think of it. And he was her hero too.
Sure, the Wonderbolts as ponies weren't that great up close once she (and we) met them. But before she was finally allowed into their academy, made sense that she wanted to be one of them.
What football position would Firefly and Rainbow Dash play in?
(American football, not soccer)
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