Out of all the flavors I've tried, I only dislike Cappuchino ((spelling?) It's too bitter for me), but I guess I'd say either Birthday Cake or Strawberry. The only place I know with Birthday Cake is Braums and we don't always go there, so Strawberry is my choice more often.
My mom's favorites are Cappuchino, Strawberry Cheesecake, and Mint Chocolate Chip. I don't think she could choose between the 3.