Years ago, I would have said females are way more ticklish. This is because I was comparing most of the females I was tickling to my own ticklishness. I have never lost a tickle fight. Later on as an observer, I saw that a lot of guys were getting their asses kicked by their girlfriends in tickle fights. They appeared a lot more ticklish than the girls.
I've seen a couple scientific studies, and the results were mixed. I think there were three indicating males were slightly more ticklish, and two indicating females were slightly more ticklish.
Honestly, I think other factors than gender play a role.