Oh. . most advertised fetish.
I re-read the OP's OP and she is asking about what is most ADVERTISED fetish.
From what see on the various sites I would have to say
TS (transsexual not table-shower). Is that a fetish? It is in my book since it does nothing for me. They get whole sub-categories on the boards.
BDSM - I see plenty of providers who have an ad in the standard category with a reference to BDSM in the text and then also have ads in the BDSM catagory. These services do nothing for me at all, I mean I don't want to be dominated by a woman. Turn that scene around however and I can really get into dominating a woman. Collar, leash, spanking, insertion, tight leather or latex, submissive crawling on the floor. (Ergo the high boots to protect her knees
) For me it is not about any pain, it is all about the domination.
Multiple participants - not really a fetish per-se , but popular and outside of the vanilla experience.
Is that answer more on point Kelly?