What an interesting topic! I always wondered. Well, I think either excessively extrovert or excessively introvert people are the most ticklish.
As a kid, I was deeply introvert and, at the same time, insanely ticklish all over. I couldn't bear being examined by the doctor : this was pure torture for me. Once, this doctor suggested my parents to give me one foot tickle session a day in order to make me "less introvert" (they never did that) : he pretended overticklishness was a symptom of deep introversion. While growing up and becoming reasonably extrovert, I became much less ticklish than I used to be.
I also remember watching an extremely shy, inhibited, full of complexes girl getting gangtickled by some other girls in a dormitory during a camp. These are probably the most intense reactions to tickling I've seen to date. Four or five mates jumped over her while she was lying on her bed completely dressed (even shoes on!), took off her shoes ans socks, held her shirt up and began to tickle her all over. What a pain! What a distress! The girl was neither laughing loudly nor screaming : she couldn't. After only a few seconds of tickling, she was breathless, almost fainting, completely unable to speak. Pain and panic could be read on her blushing, sweating and soon crying face. In the same time, she was squirming like a worm with an amazing energy and strength : this was an astonishing metamorphosis!
So, I think that extremely introvert people are extremely ticklish but that tickling is often very painful to them, due to a too high contrast between their usually censored emotions or expressions and the sensations and reactions tickling triggers on their body : they suddenly feel discovered, afraid of being forced to see and let show aspects of themselves they usually try to repress.
In the opposite, I remember one tall, thin, hysterical kind of "grasshopper woman" going nuts at the slightest touch on her soles or ribs. This woman, probably one of the most funny and extrovert I've ever met, was once gangtickled by me and some friends of mine. God! This one not only squirmed like a worm but also laughed outloud.
And in this case, tickling really seemed to be as pleasure as torture to the ticklee (she later confessed to having enjoyed the session).