While I am very ticklish everywhere, I would have to say my feet are the most ticlish HOWever, thighs,knees and sides are a very very close second. Being tickled on once place makes the rest of me more ticklish when tickled elsewhere.
Until last year, I thought it was my belly....but since being tickled by my girlfriend while tied, it's definitely my feet. She uses those long fingernails on the balls of my feet and under my toes and I go totally insane (but of course I always pay Traci back in kind....and her bare feet are much more ticklish than mine (from her soft heels to those cute little toes 😀