This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / One Flew Over the Cuckoo's Nest
Bromden's…dream…hallucination…reality…whatever that thing was when he didn't take his pills one night. It's a strange, Silent Hill-esque sequence where a wall opens up to reveal a large mechanical slaughterhouse where workers move patients around on hooks and slice them open, where upon they bleed rust and ashes. In the true horror fashion, the old Chronic mental patient that Bromden saw get sliced open and mechanically bled out really is dead the next morning.
The description of the patient who was an angry lunatic before, but became an empty shell after his lobotomy. His eyes are described as being like burnt-out lightbulbs...
The rape of patients by the orderlies... just the knowledge of the patients, who are largely helpless, being forcibly raped while being completely unable to defend themselves (and this is going on with all the nurses knowing) is utterly scary.
The Electroshock scene, it actually looked like they shocked Jack for real.
Nurse Ratched. As the Rotten Tomatoes Show put it, she has so much power in this movie that you get nervous just talking about her!
Yeah, all the horrible abuse and cruelty against the patients in this? It was, for all intents and purposes, MILDER than what occured in reality in the era when the original book was written.
Stuff that was occurring on site, as the film was being made, was pretty terrible. Such as the hundreds of cremated human remains that should not have been in the building. To add to the horror, some of them were -lost-.