I'm the sort of writer who will get 1000 characters to describe himself, and waste all of it to explain why Rarity is his favourite pony, instead of talking about his own work.
Rarity, more than any of the other mane six characters, exemplifies what I think My Little Pony: Friendship is Magic gets very right. She's a wonderful example of a pony that defies traditional gender roles, without entirely dismissing them.
It's easy to be critical of a female character that has fashion as a primary interest because it's such a stereotypical interest. But Rarity isn't just interested in fashion. She works very hard as a fashion designer. She gets frazzled not because she can't find the right outfit for the big dance, but because she's trying to design five outfits that will please five very different ponies in too short a period of time.
Rarity is very passionate about what she does, and while her ego occasionally gets in the way, she obviously cares about other ponies and their needs.