Long post incoming. I've watched a lot of the movies mentioned in this thread, some thoughts:
Everyone is gonna say some shit like Salo, but I present to you a film that is not unlike the content on this website: Be My Cat, its a romanian film, so its likely based on a true story, about a guy that wants to impress an american actress.
you can watch the whole thing on youtube, not a 10/10 but its disturbing in its own right.
https://youtube.com/watch?v=nzUXtaI7xoI
I thought this was quite poor. I didn't find it scary and it felt more like a black comedy (or at least trying to be one). Clearly really low budget and didn't really have a resolution, it just sort of ended as I remember. I did think the scene where he's
cutting up a woman and then when caught plays the whole thing off as a movie being filmed was sort of amusing and the scene where he
chokes a woman to death with belt iirc was a bit disturbing but that's sort of it. Speaking of Salo, I liked that a lot and but I think most people missed the political commentary and just watched it simply because it's a 'fucked up' movie. I didn't think it was that graphic either.
Angst (1983) will always win this competition...I invite all to check it out.
I always wondered if the makers of Silent Hill watched this, it had some very creative camera work. Good film, but there's not a huge amount to it.
Henry: Portrait of a Serial Killer and The Golden Glove stand-out because they pull no punches realistically portraying their serial killer protagonists as cruel, grimy, stupid, and hateful. The Golden Glove is more graphic but also has some funny dark humor segments, while Henry is genuinely one of the most oppressively bleak movies I've ever seen.
I felt a bit let down by Henry because I didn't find it as horrific as I was led to believe (with its reputation) but it was a solid movie and Michael Rooker was very good in it. I also liked how they didn't go with a
happy ending.
There's something peculiarly cursed about 70's/80's animation. Ever seen the Soviet adaptation of Bradbury's "There Will Come Soft Rains"?
Better quality here:
You would enjoy
Dead Man's Letters
Can't really get disturbed from movies, but Begotten could be for some people, if they don't mind silent films that is.
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Last House on Dead End Street. A guy gets out of prison and makes snuff films as a means of revenge against society. Featuring a deer hoof blowjob scene.
All we have left of it is a 70 minute or so mess of a film that's been cut to ribbons, and what's there isn't really worth watching imo. The original version was 3 hours long, and it was filmed with a camera the director personally got from Nicholas Ray. I'd recommend people look up the backstory to this film because it's very interesting, but watching it in the form it's in isn't worth it. At least the murder scene at the end was left intact; I remember reading that people thought it depicted a real murder, which added to the mystery of the film.
My personal pick for most disturbing film is Isao Takahata's animated feature Grave of the Fireflies. I don't call it anime or think of it as anime because anime connotes entertainment and 'Graves' is...something else. The film is notable for its utter absence of hope, as in there is none as far as the eye can see. You see two starving war orphans starve. I think the director was going for saddest movie ever made by man.
It affected me profoundly.
I seem to be in the minority here but I was really unimpressed with this film. I remember barely anything happening in it, and after it was over I was thinking 'was that it?. I've seen My Neighbour Totoro as well and I didn't think much of it either. Actually, I can count on one hand the number of anime films I've seen which actually lived up to the hype e.g. Perfect Blue; they tend to get overrated.
Martyrs, from 2008
I watched most of that movie literally covering my eyes, and it was still hard to watch because of the very realistic screaming and sound editing. The premise of the movie is effectively horrifying to think about as well, because you can 100% believe that there are nutcases out there who would do this.
I think because the violence in the film was so constant it desensitised me to it, making it less disturbing than it could have been. Definitely one of the better made 'extreme' horror films out there anyway.