The Kungaloosh from the Adventurer's Club is my favorite. It's not too sweet, but you really can't taste how potent it is. Yummy!
I also really enjoyed the traditional flavored frozen margarita in Mexico at Epcot. I was expecting some insipid fake lime taste, but it was actually quite good and authentic.
The Kir Royale at Chef's De France is also very good. Nice and light before dinner.