Scootaloo is my favorite. Rarity is up there on my list, though.
But look at Scootaloo. She's a daredevil, she loves music, and she's pursuing her cutie mark with such perfect dedication. I really love this character, I really relate to her.
It's been hinted at a few times that her cutie mark might be playing a musical instrument or dancing. I greatly appreciate both, being so musical myself.
Additionally, she has a 1980s New Wave-esque quiff haircut. And it looks great on her!