This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Creepshow
The end of They're Creeping Up on You! is absolutely horrifying with cockroaches pouring out of an old man's mouth, then out of every orifice, and finally exploding him from the inside out. Also an example of Nausea Fuel and Paranoia Fuel.
The DAMN COCKROACHES. The worst part? No special effects. They're all REAL!note If it's any consolation, it's a mix of living bugs, safely dead bugs, and things like raisins that look sort of roachy to fill out what looks like a big horde.
Tom Savini (who has a massive bug phobia) mentioned in a book on effects that when one of the cockroaches touched him, he teleported across the room. Not really a joke - he has no memory of physically crossing the room.
This scene is on Bravo's top 100 scariest movie moments of all time.
Also, Richard's revenge. Being immobilized in sand while the tide gets closer and closer to you and ultimately covers you.
Harry threatening Richard is more scary in that sense too. Especially when he manages go through with it after his death.
Not to mention the fact that it escapes from its crate at the end. And it's angry, and it's still hungry.
The ending of The Lonesome Death of Jordy Verrill is as unsettling as it is sad. Poor Jordy has been completely consumed by the alien vegetation, to the point where he can barely move. He has nothing left to do then put himself out of his misery with a double barrel shotgun. The worst part is that the vegetation is still growing, and heading for larger civilization.