What was the most disturbing movie you ever watched?

A Serbian Film would have to at the top of the list for sure.

For me it was Eraserhead.

The Vanishing (Spoorloos) (1988)

SAW
Hereditary
Ratu ilmu hitam ( its an indonesian movie. If u want to watch this movie, i highly recommend it )

It has to be Fire in the Sky.
It's the supposedly true story of Travis Walton's alien abduction.

Showgirls. Absolutely frightening.

A few others from my memories (less disturbing to me than the 1st I mentioned, but still):
C'est arrivé près de chez vous / Man bites dog Maybe less shocking decades later, but when it was shot in the early 90's, it was considered very hard to watch. A must see for people who love twisted jokes.
These ones are also interesting, while less "top" candidates:
Valhalla Rising Mads Mikkelsen is disturbing by himself :) There is something weird in this movie that makes watcher uncomfortable.
The Lobster Not much visual violence in this movie, but I found the atmosphere and story very disturbing. Maybe a personal thing.
Bunker Paradise Story and actor's play make watcher uncomfortable. This is about a group of ultra-rich young adults who knew ultra-wealth all their life, so they are so bored that they are sad, empty, and mean.
Calvaire The story of a man whose car break at the wrong place during a Christmas trip. Wrong place, indeed !

personally,i found this disturbing as well...https://en.wikipedia.org/wiki/Existenz / https://www.imdb.com/title/tt0120907/

A Serbian Film
Audition

damn i forgot the name but it was about few friends on video conference call and one of them found a laptop at a coffee shop, steals it and next thing you know, every one is getting killed one by one, at the end, the guy just runs for his life, riding a bicycle and gets hit by a car.. all dead :\

Hostel and/or Hostel 2 - I don't remember...

The divide, it shows the worst of humanity.

August Underground -- it's a trilogy of movies about serial killers, filmed "Blair Witch" style.

Coraline

Thanks a lot teacher from second grade.

Have you seen Begotten (1990)? Pretty messed up.