My very first post!
My single most ticklish spot are the backs of my knees. I will be honest, I am a virgin when it comes to being tied up and tickled for long periods of time (though...::shiver:: that sounds good to me), so I haven't really explored all of my body. However, I am in a co-ed frat, and I had an experience with two of my "brothers" pinning me facedown, one straddling my hips and the other one sitting holding my ankles down with his thighs and just skating his blunt fingernails up and down the backs of both knees over and over again for a good two or three minutes straight. Never changing it up or doing anything fancy, but the way that it just didn't stop or vary and I couldn't move my legs at all had me...beyond hysterical.
When they finished, I smacked them good naturedly, but then I had to leave.