Flat out, the most ticklish person I've known was the second girl I mentioned in Biscuit's massive thread. She was a tiny blonde (about 5'0") that was kind of a hippy chick. I could poke her anywhere, and she'd close her eyes with a helpless grin. If I gently stroked her sides or pits, she'd squeal and try to squirm out of my grip. She was the silent laugher type, and any amount of extended tickling would cause her to squeeze her eyes shut and open her mouth in a contorted smile. Just touching her small, soft feet would cause her legs to twitch and her toes to curl. Giving her a foot massage was a challenge because she couldn't keep her feet still, and she would squint her eyes while trying to stifle giggles. By the time I would rub her toes, I'd have to get a good grip on her feet, and she would squirm uncontrollably.
My first girlfriend was almost as ticklish as this girl, but she was mostly ticklish on her feet, pits, and nipples. I'd have to sit on her calves whenever I got her feet, because she'd kick her legs and squirm so much. For those of you familiar with Phoenix from Magic Touch and Tickling.ws, this girl reacted very much like her, and her feet looked a lot like Phoenix's as well. God... how I loved that helpless laughter....