I agree with some of the others. It probably does mean different things to different people. Kayakid's concept of "if you had only one to see" makes sense. I also think that having multiple ATFs simultaneously is oxymoronic, or perhaps only hyperbolic. I think, like many things, it becomes a shorthand, or label, if you will, but not in a negative way, as many labels are deemed. When someone uses the term, I take it to mean that he had a great connection, a great experience. So if a provider is someones ATF, that gets my attention and makes me think she must be really good in some way.
As a relative newbie, I have not yet used that term, and I'm not sure if I will. I am very selective and try to do lots of research about whom to see. Fortunately, all of my experiences (other than my first, a ripoff) have been good. A number have been very good, and a handful have been exceptional, as in I could see her very often, funds permitting.
As with the top 100 lists, many people like rankings, and the ATF is presumably #1. But some things are not amenable to ranking. Of my top experiences, one may be better in a particular regard, while another may seem better in a different regard. The important point is that the experience was enjoyable and ideally unforgettable. I can happily attest to the fact that a date--and that's really what it is--with Megan is remarkable and memorable in the extreme. Thank you, Sweetie