I have a special place in my heart for Cotton Candy since she's the first pony I had, and I remember desperately wanting Blue Belle as a kid, but my vote goes to Snuzzle for her unique coloring and adorable name.
I'm afraid I'm going to have ot be a follower on this one and say Minty along with the dozen other people who already have. She's really the only one of the CPs I even like. Such pretty colors.