Honestly, I have all 7, and I could never pick a favorite from the group. They're too endearing.
I think I did a similar poll on the Arena a while back, only with all 7 characters, and if I remember right, Chocolate Chipper got the lion's share of the votes. Gotta love a chocolate pony! But Pretty Pop is one of the few yellow ponies that I obsess over. She sticks to my mind like bubblegum! So I could never choose.