The stomach region alone is tantilizing, but the bellybutton always draws my utmost attention. I'm slightly sadistic, too lol so that doesn't help. Especially if I have them tied down...
I just love to see a person's reactions as my fingertips gets closer and closer to that small indentation. They know it's going to tickle like hell, and they're sort of afraid of it, but still can't wait to feel the first touch!
I personally find that "innie's" are the most sensative and I think it's because of all of those little folds at the very bottom hehe that's one of my favorite places to hit.
I haven't had the chance to tickle another female's bellybutton yet, but maybe I won't have to wait much longer...