When I was a kid, we invariably went to Baskin-Robbins for ice cream, so some of my longtime favorite flavors are from B-R: Mint chocolate chip, their World Class Chocolate, which is like a mix of light/dark chocolates, and their Peanut Butter 'n Chocolate. (I've still never had a better peanut butter/chocolate ice cream anywhere else.) My parents' favorite B-R flavor was Pralines 'n Cream.
Sadly, there are no B-Rs in my area, so I don't get to go there much anymore. Around here, my favorite flavor is Mocha Chip (coffee/chocolate ice cream w/chocolate chips).
In sundaes, I usually just go with vanilla, as it's the best contrast to sauce toppings.
Once in a while in New England, you'll find places that serve lemon custard ice cream, or some variant thereof. Unusual, but delicious.