This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Night Shift
"The Boogeyman", full stop. An eponymous creature that stalks and kills a young man's three infant children, who hides in the closet and moves around at night in such a way that it wants you to hear it. Billings tries to leave the old house, hoping to escape it but the boogeyman tracks them down and takes up residence in their new closet. Brr.
The ending is even worse, with Billings discovering that the psychiatrist he has been telling his story to the whole time is the creature in disguise.
So nice...so niiiiice
Elizabeth's nightmare in "I Know What You Need." Briefly, she's laying in an open grave, unable to move or speak. Her boyfriend appears on the edge and tells her to marry him, or else. When she doesn't respond, he says, "It's or else, then." She hears a bulldozer start up, and realizes she's about to be Buried Alive. But suddenly another man, someone she had just met and befriended (in her real, waking life), shows up and stops it from happening. He reaches out to her, she finds she can move again, and he pulls her out of her would-be grave. She looks down at her footing as she climbs out. When she looks up again, her friend has become "a huge, slavering timberwolf." Elizabeth's nightmare undoubtedly caused a lot of nightmares.
The Nothing Is Scarier description of the painting the narrator sees in the abandoned church in "Jerusalem's Lot". He finds himself unable to describe it in any great detail in his letters, only that it was a grotesque, unholy version of the Madonna And Child, done in the fleshy style of Peter Paul Rubens, and that shadowy, half-formed creatures reveled in the background.
A good deal of the village of Jerusalem's Lot, even before its true nature is revealed, especially the inverted golden cross in the church.
The ideas behind "Gray Matter." To think that you could be transformed into a fungoid blob that eats dead maggot-ridden cats and those unfortunate enough to cross its path, by such a simple and common pleasure as drinking a can of beer, is bad enough. But, arguably, the most nightmarish thing of all? In the words of Richie Grenadine: "But it don't hurt. It feels....kinda nice." If your transformation were painful, you'd at least understand that something bad was happening to you and you would have motivation to fight against it (whether or not that fight would be successful, or whether you'd end up caged in some scientist's lab, is another story). But it feels good!Can you imagine anything more horrible? Chug-a-lug, everyone! Drink 'er down!
In "Gray Matter" we also hear the story—recounted by our first-person narrator—of George Kelso, a Bangor Public Works Dept. employee who went down into a sewer pipe to do his job, "laughing and joking like always," and came up fifteen minutes later "with his hair dead white and his eyes bugging out like he'd just looked through a window into hell." Then he punched out from work, went to a local bar and started drinking. In two years, he'd be dead. The narrator says that George never so much as hinted about what happened, except once when he was drunk out of his mind. He asked a friend "if he'd ever seen a spider as big as a good-sized dog settin' in a web full of kitties and such all wrapped up in silk thread."
The main characters from "Night Surf" burning a man alive as a sort of quasi-sacrifice. He was half dead from the superflu, but still.
The child vampire from "One For The Road", and the way she mesmerizes the main character Booth. Only the timely intervention of his friend saves him.
Renshaw's death in "Battleground", getting incinerated by the to-scale nuclear bomb that was included in the package with the sentient toy soldiers. It wasn't a big bomb, but big enough that it wiped out everything in Renshaw's apartment floor. Including him.
The story of the haunted refrigerator in "The Mangler" (which brings to mind a similar fridge—Patrick Hockstetter's killing box in It). And the Downer Ending of "The Mangler," where it escapes the laundry and goes on a killing rampage. Nothing short of a bazooka is going to stop the demon-possessed thing which used to be a Hadley-Watson Model 6 Speed Ironer and Folder. And who knows? Maybe even that wouldn't work.