It's really hard to tell. I can tell you that on other sites, I sell a broad range of subjects from my port daily with some star-players streaking ahead of the rest.
On FT, I sell a small proportion of my port, and only those same images sell over and over again. They're not ones that do well on other sites either. My best seller by far on a multitude of other agencies sells nothing at FT.
The only useful info I can glean from my own experience is that Europe-centric subjects seem to do a lot better there.