Mine is the belly. It’s just a wide open area that is easy to tickle. My belly is crazy ticklish and I can’t stand fingers lightly brushing up against it. Bare belly is even more ticklish.
I love to tickle legs since a girl I liked when I was a teenager was super ticklish on her knees and thighs that's been my favorite place tickle since then, but I have an armpit fetish so armpits are a very close second.
Inner thighs, knees or in between my ribs...yikes. Also once I was tied down on my back for super light tummy tickles (which I thought would be easy to take)...I thought I might actually break the table. Not sure if it was because I was laying flat or because I couldn't move...but I was ready to give up all my secrets.