This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Men in Black
The live-action film:
There. Izzat better?
The alien at the beginning of the film freaking out when the border patrol officer sees him.
Edgar's giant bug form.
Edgar's skin, decaying throughout the film, is arguably more unsettling than his real form.
And speaking of skin, the idea that he skinned the poor guy alive with his bare hands.
The scene where his victim's wife comments on the looseness of his skin, and he stretches his face back against his skull.
One minor one from the film's novelization; when going to Jeebs' pawnshop, the future Agent J warns Agent K about how his car is likely to get stolen. K brushes it off, and mental dialogue reveals why he's so nonchalant; his security system is alien design. What does it do? It quietly and casually disintegrates anyone who tries to pick the lock! By the time that they step through the shop's doors, about five would-be carlifters are dead, and K just smirks to himself that this neighborhood is going to get safer and safer the longer they take inside.
Kay decides to get his gun back after the bug swallows it...by getting the bug to swallow him.
The Skraaldians from The Long Goodbye Syndrome end up being Paranoia FuelUp to Eleven. They're a shelled slug-like race that comes to Earth to multiply, as they like the warm climate in the sewers. However, if any single one of them is killed, the victim will vomit on the killer. The vomit then permanently marks the killer down to their DNA as a target of revenge for the entire race due the race being a psychically linked Hive Mind. Worse off, they can hide in the body of other aliens as parasite and burst from their chests like Chestbursters from "Alien" to get to their victim, and masquerade as other species as something as innocent as a human hot dog vendor before they strike. If you kill two or more, they will slowly put you in a large vat of soup at temperatures of 3000 to 4000 degrees to melt. They're so bad that Kay was willing get stung by one and swell up and turn the color of rotten eggplant for a week to avoid their wrath. As Kay describes to Agent J after he killed one of them:
Kay: Skraaldians possess a hive mentality. They're psychically linked. They saw what you did. They know what you look like. They know where you live. All of them.... Listen, Slick. This is a whole planet of dudes dedicated to your demise. Four billion assassins locked on to your DNA, thinking about you every three seconds.
Kay managed to get Agent J out of the death hit using an Arquillian robot suit decoy who owed him a favor, but if he hadn't J would have been a dead man for certain.