I'm new too, but the feeling I'm getting is different categories convert differently depending on the sponsor. Popularity is different from conversions as well.
Once you have a few sponsors setup, you'll normally see a list of top converting sites vs. top selling sites. Cam and dating sites seem to need less traffic to convert, but you'll get more volume with teen or milf.
Like I never gave black lesbians much thought until I signed up with pimproll. It's their top converting site. Other sponsors will have tranny as their best converting.
So I think listing categories by popularity would be moot without factoring in everything else in areas I'm still learning myself.
If you just want a general idea, use the
google keyword tool and type in "porn", sort by number of searches and start digging from their.
Try to find a niche you think you can sell, then look for a sponsor that has that as one of their best sellers. You can also visit whopaidme.com and
http://www.webmasterbrotherhood.com/ to get a feel for popularity.
Like I would have a hard time selling gay personally since I'm straight. It's hard to write about something when I have no idea what might get a that audience excited. I do well with virtuagirl since I have a software background and hence can write something more tuned to what someone who would install that software is looking for. For example, "best freeware", or "adware free", or "uninstall anytime". Little things that someone might consider before installing something.
I hope many others will chime in on this thread, because I'm only guessing at this point. I don't have enough experience or data to back up my statements.