I don't have just one. It depends. But I really like black raspberry, butter pecan (especially with lots of nuts!), chai, and alcoholic ice cream (Bailey's, Kahlua, et cetera). I also enjoy frozen yogurt - the tangy kind, like Yotango at JP Licks, or what they have at Berry Line, not like TCBY. However, I have developed something of a hatred for sorbet, because it's often the only dessert I can have and I never liked it much in the first place. Imagine people randomly telling you that you want a dessert you don't especially want, because "it's really very good," and they do it over and over... unfun.
ETA: I forgot to mention that when I am craving chocolate, coffee ice cream is a pretty good substitute.