Well I guess this would have to qualify as a good question DK! Shucks, after I decided to reply, I realized there were four pages of replies, so I had to start taking notes as different folks brought up things I wanted to respond to! LOL
Personally, I'd have to echo what MTJ and DVNC wrote about being drawn to the place that gets the best reaction. Behind the knee or behind the eye lid, I'm for the reaction. Yes, eye lid! LOL I know a girl who laughs and swears it tickles her eye ball when you point at her eye... so guess who always wiggles his finger a few inches away from her iris to get her giggling? Sandman, Mimi and Tcrew brought up good points about the benefits of finding new, less "run of the mill" spots. I've found many folks have this inherent phobia of the traditional "ticklish" spots, so if you even LOOK like you're going to touch a barefoot or underarm, bloody hell breaks lose. But if you're holding hands and lightly tickle the wrist or slide fingers down the shin, I've found it can be viewed as far less "threatening" and even received with surprise and pleasure.
Nevertheless, while my predilection for probing keeps me from having a favorite area to target, I DO have a female body fetish. Maybe it's because of my love of tickling, but either way, I feel like each body part has it's own unique tickle appeal. In some places, like Dave2112 mentioned, the connection between tickling and sexual arousal is stronger, i.e. breasts, lower abdomen, lower...ahem! Then there are areas like the feet, where the expression of wiggling, curling toes can have an almost hypnotizing effect. The armpit always seems such an intimate spot to me. The mid-section allows you to have a great deal of body to body contact while tickling. For me, generally tickling is about the expression of affection, so I could go on and on for each spot. I feel there are a million ways to tickle and a million ways of tickling.
Of course, even after all of this, I think the input of the 'Lee is invaluable! (Thanks Mims & Kims!) As DVNC mentioned, often time, they'll let you know what works the best and where! LOL Maybe one of these days I'll be able to find that spot on ya Kimmy!