I think, for me personally, it's either "Salò or the 120 Days of Sodom" or "Come and See"
"Salò or the 120 Days of Sodom" strikes more with it's sexual abuse, brutal killings/executions and the not shying away from showing everything
"Come and See" on the other hand just gives me anxiety, the amount of brutality, both physically and physically endured by the protagonist, turning him from a joyful boy into a empty shell is heartbreaking
So, for me it's between those two, but if you like, I have a list on letterboxd where I listed the disturbing/extreme/controversial movies I have in my collection by how I feel about them For those interested: Ranking List: https://boxd.it/ln2po