In a Glass Cage is a Spanish Film from the 80s about the cycle of abuse (among many other things but that's the driving theme). It's not outright gory or sick like something like
Salo but it really got under my skin, for some reason. The whole thing just felt really unnerving and it's one of the few times I've felt genuinely uncomfortable watching a movie. It's free to watch on
Tubi if you want to (legally) check it out (and should work fine even with an AdBlocker. Tubi is really underrated imo, but that's off-topic).
Movies like
8MM and
Hardcore about shit like snuff films and how seedy the porn industry is have always gotten to me too. Out of those two
Hardcore is a lot more visceral imo. I like both for what they are. Wouldn't call them abjectly disturbing but they've always gotten a reaction from me personally.
Shinya Tsukamoto's Tetsuo movies (especially the first one) are really good body horror. I haven't rewatched them since my first viewing because they literally hurt my brain. May or may not be considered disturbing depending on individual's tolerance for that kind of stuff.
Speaking of Jap shit
Pulse and especially
Cure are both directed by the same guy, and extremely good movies, on top of being pretty disturbing. Again, not so much in the sense that something like
A Serbian Film is; these are both very quiet, contemplative movies that leave you alone with the subject matter that you feel like you shouldn't be seeing. I like both and find them really disturbing for different reasons:
Pulse is very prophetic in regards to the Internet and how it inevitably evolved (or devolved) and
Cure is just an eerie, evil movie (in a good way).
I know it's A24shit but I also have to mention
Hereditary as the only movie that I've seen in theaters that genuinely unnerved me on a really deep level. I was seeing shadows/figures in the corner of my ceiling for like a week lol.
Also more contemporary,
Skinamarink is really,
really hit-or-miss with people but I thought it was effective. I think it was mostly the fact that it was about two helpless little kids stuck in a fucked situation they had no possible way of getting out of, rather than the filmmaking/scares or anything. I hate kids getting hurt in movies, even scary ones where it's to be expected, so it really got to me - a particular scene at the end especially.
Those are just some off the top of my head that haven't already been mentioned (at least not that I saw) in this thread. Some are kind of subjective as far as how disturbing they are but I think they're all movies worth watching one way or another. I think to a certain degree they all stand on their own without relying on the shock value of how disturbing or disgusting they are.