The best mainstream tickling scene I ever saw was when Patrick Duffy tickles Victoria Principal in the 1984-1985 season finale of "Dallas". After their characters, Bobby and Pam Ewing, spend a romantic night together, after being divorced for several years, they decide to remarry. They awaken in the morning, and Pam says to Bobby "I remembered everything about you". To which Bobby replies "Is that so, including the fact that I know you're ticklish". Pam then says "Oh, not very". Bobby then turns around, pins Pam down on the bed, and gives her two good goosings of her sides and ribs, and Pam is laughing hysterically, and writhing in his grasp. Back then, I had the major hots for Victoria Principal, and even though I wasn't into tickling like I am now, I can still remember loving that scene.
There have been other good ones, but the Dallas scene was my favorite.
Mitch