This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Requiem for a Dream
The whole movie could apply to some, though most of it falls into Tear Jerker territory.
The real NF here is Sara's descent into madness over the course of Fall and Winter. This sequence gives us such gems as Sara's POV of the world, which is menacing in every possible aspect, a refrigerator coming to life to eat her complete with loud, startling banging sounds, a TV personality emerging from the TV as a bizarre being made of static and mocking her, and the simple, horrific image of a completely fucked up Sara walking down a sidewalk in broad daylight as people pass by so quickly they are blurred and look like ghosts. That last one also serves as the first image seen during the Winter segment, just to make sure you knew what the last third of the movie was about to be like.
Stripper: So what do you want us to do now?
Uncle Frank: Ass to ass. [Someone holds up a greased double dildo]ASS TO ASS!
The shock therapy, and the sounds accompanying the scene. It's really played as Electric Torture, and is absolutely agonizing to watch.
Appropriately enough, the corresponding track to this scene on the soundtrack is titled "Meltdown".
Anything in the "Winter" suite of the soundtrack, especially the aforementioned Meltdown.
"Lux Aeterna" is as creepy as it is awesome. More creepy, actually (except the orchestrated "Requiem for a Tower" version, which is just awesome).
Marion having a dollar bill stuffed into her mouth during the Madness Montage. There's something succinctly horrible about a barely-conscious woman being gruesomely violated by a stranger, for a crowd of jeering strangers, and being fed a single dollar from an anonymous, sweaty hand. It's a hell of an image to express how worthless her addiction has made her.
The whole idea of that part of Marion's arc is horrible. This is a man who has an education for the express purpose of understanding, analyzing, and learning how to treat mental illness. The "You smug fuck!" scene drives it home; the entire reason for his getting involved with Marion in the first place is because he knows that she's a weak-willed dreamer. He knows damn well that she's at risk for addiction. He's violent with her because he's been waiting the entire time for her to whore herself out to him, because he knew that's what she'd eventually end up doing... and like most abusers, once he knows he's got her once, he knows he's got her for good.
The scene where Harry and Tyrone are both crying out and screaming for help, accompanied by harsh camera shaking and grating distortion noises that intensify as their screams get louder.