I know I said the same thing about Rarity, but Dash is in very close second for me, and I don't think that one thread did her full justice.
1. Ashleigh Ball is a fantastic voice actress. It's easy to overdo the whole tomboy thing, but she's able to balance all that arrogance and pride with insecurity and humanity, or whatever the equivalent would be for ponies.
2. She has had the most character development out of all the Mane Six, even more so than the one who grew new limbs.
3. She's usually the best part of even the worst episodes.
4. She embodies her element better than any other.
5. Rainbows rock.
6. She has a clearly conveyed goal that she's working towards over the course of the series. You want to see her become a Wonderbolt and to live her dreams, and thankfully, the show is actually allowing development on that front.
7. Incredibly cute. The mane, the voice cracks, the fangirl personality.
8. She's just so freakin' badass. She can clear skies in ten seconds flat, has a black belt in karate, and is making the laws of physics her punk on a daily basis.
9. FlutterDash is Best Friendship.
10. There's ton of potential regarding her character development. She still has to learn how to become a good influence for Scootaloo's sake, she still has to become a Wonderbolt, she's always undergoing some surprising change in personality or thinking, and she still has to finish that Mary Sue novel.
11. Much like Rarity, Dash's character is a great blend of likable, admirable qualities and rough edges and sometimes negative attributes. She's stubborn, not always thoughtful of other people's feelings, greatly self-assured of her own coolness, and her fear of being seen as weak or emotional can be all absorbing. But she's also courageous, loyal to a fault, and though it's rare, she can be deeply affectionate.
12. She inspires the best fan music.
GO RAINBOW, GO RAINBOW, GO!!!