Florida
I've always thought that was a silly concept. Something born out of the need to quanitfy every freaking thing in the world.
Even in my short time in the hobby I've been afforded the pleasure of spending time with some absolutely amazing women (both reviewed and not). Every encounter was great in it's own way. Were some visits "better" than others? Well of course. It all depends on what kind of mood you are in, what you are looking for, and if there is some kind of connection, whether "real" or not. It's perception.
For me...it would be like picking an ATF food. Not possible. If the circumstances are right, my current craving satisfied, and the taste is fabulous...then that's my ATF at the time. (gee...I guess that could apply to both things, couldn't it...LOL)
Now I just need to find my next ATF for tomorrow.... 
it is a loaded question. How many guys are going to get up on the firing line and name ONE lady on this board. I would, however have rather seen NO responses than these. To me the ATF is a lady who just totally stands out in you memory for one reason or another. Someone you would drop anything you're doing to see. Although there is no lady that I've seen, that I wouldn't see again, (and most I have repeated with), there is one that is just etched in my brain. It had nothing to do with looks, build, menu items, etc. It was totally because of the flow of the time spent together, absolute total chemistry, and just seeming to know when and how to turn it up a notch!
Like the rest, I won't name names. Suffice to say there is one. And that takes nothing away from any others I've seen, as I have been very lucky to meet and spend time with some VERY special ladies.
but since no one else is playing by the rules, why should I. Anyways all of you who can't decide upon one, either can't make a decision or you haven't found one yet.
having ONE ATF is a stupid, senseless idea that someone in the hobby came up with. To me All Time anything denotes one. In sports it's figured by stats, the hobby is more nebulous. You can see a lot of ladies, and have a lot of favorites...I know I do. But if there is not one lady that has made your head spin like the exorcist, you have not met your ATF yet...plain and simple! I knew 1/2 way through the first date this lady was totally different...not better...not worse...just amazingly different.
So for you guys who can't seem to come up with one, keep looking. When the high voltage clicks on, you'll answer this question differently next time. It's not something you need to decide upon, it's something that just happens.