Pinkie Pie is my favourite because she is basically me. However she's been mean to Fluttershy a few times and that made me really sad. I've worked very hard on being sensitive to people's issues, especially social anxiety, and it was disappointing to see her scaring Fluttershy.
I don't know why, because it's totally out of character for me, but I also really like Rarity. I'm thinking it's her generosity (and the accent).