Grotesque - a young couple are kidnapped by a mysterious doctor, who subjects them to torture in order to satisfy his perverted desire for excitement.
I think this ended up on my watch list because it was the last film to be flat-out refused a certificate in the UK; no resubmission with cuts, no possibility of compromise, do not pass Go, do not collect £200. Naturally, that's the kind of thing that grabs the attention of tragic figures like myself, but it's actually fairly meaningless. There are hundreds of films that wouldn't be allowed a UK release for various reasons - the only reason those films haven't been refused a certificate is that nobody was stupid enough to bother submitting them to the BBFC for release in the first place. So, Grotesque holds the current record for having the daftest rights holders in the UK. Bravo.
The thematic reason for refusal is pretty clearly sexual humiliation, although the relentless wallowing in senseless misery would no doubt tick the old 'deprave and corrupt' box on its own. Grotesque is part of that peculiarly Japanese subgenre that can accurately be called 'torture porn'; films in which there is no real plot, you're just there to see people being tortured in as realistic a fashion as possible. No doubt there's a fair amount of artistry goes into the special effects, but you see one of these things and you've seen them all. And probably wish you hadn't bothered in the first place.