With an eye toward self-publishing, what would you consider to be the most marketable genre/sub-genre/meme in the fiction novel-writing world with the highest potential for profit?
If you write fabulously well and/or write stuff that grabs an awful lot of readers for whatever reason, then genre doesn't matter. Blockbuster is blockbuster.
If you write competently and don't care what you write, well-written, well-researched non fiction is a lot easier to sell than fiction in any genre. (I speak from experience.)
If you write less than competently, you're pretty much out of luck, although (speaking from observational experience: google "dinosaur porn") badly written, self published erotica seems to sell better than one would expect.
At the moment, there seems to be a very large audience for well written erotica, so if you plan to self publish that seems to be the most lucrative genre.
Having said that, though, you mentioned "profit". There are incredibly long odds against becoming one of the few really huge-selling names (S King, D Steele, JK Rowlings, N Roberts, etc). So, you're looking at 'normal' writer income: in which case, profit has to be balanced against cost. What's a writer's time worth? To some people it's 'nothing', in that they'd just be watching TV or whatever if they weren't writing. To others, it's 'what I could earn elsewhere': if they could get a weekend job earning $20 per hour, then they'll only write if they can guarantee someone will pay enough for their prose for it to work out to >$20 per hour.
Where does 'profit' start for you?