Interesting question, but I think I'd have to say looks. For me, there is a sexual aspect to tickling, so I have to be attracted to her or I'm not interested at all.
At the same time, she has to be ticklish, but then again, I like to believe all attractive women are ticklish (a little healthy self-delusion never hurt anyone.)
As far as personality goes, I like to tickle all types, shy & demure to totally bitchy.
My feelings toward the ticklee can vary, there is a lot to be said for tickling just for the sake of tickling!