Well, in my experience, i've encountered only a handful of women that weren't extremely ticklish on their feet, given that they are ticklish in general of course. More than half of them were most ticklish on their feet, about 2 out of 10 on their sides and another 2/10 on their underarms. So, generally, there is NO rule that can tell us which spot is the most ticklish because many factors define ticklisness and a small change in them, affects ticklishness too.
As a matter of fact, i'm growing to believe that the tickler himself plays the most important role. It's a fact that most ticklers, even amongst video producers, don't have a clue about how to tickle the feet. In order to get a laugh from tickling the feet of someone, you need to try far more hard than if you tickled his/her upperbody. That's why we seen hairbrushes and toothbrushes coming into use. Upperbody tickling is much easier and that's why we see some good reactions there.
Another factor that makes feet unticklish sometimes, is that they are a spot "sinned" by common belief. Many girls feel uneasy when someone touches their feet, they get anxious and therefore block their ticklishness. Also, wearing shoes and socks all day makes you react completely different to a tickle than if you were barefoot. From my experience, if you tickle my feet right out of my shoes, they're dead. If you tickle them after i take a shower, it's the most unbearable thing on earth.