Probably for me, it depends... see, it matters how somebody tickles... thats kinda a weird one, isn't it? Like... I know people who ARE feet ticklers... they're lesser skilled in tickling say, my sides or something like that, but are sooo skilled at tickling feet.
On the opposite side of the coin, people are better at tickling sides, or armpits, ect.
but my favorite spot has to be my sides, probably.