This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / An American Werewolf in London
If the Transformation Sequence doesn't get you, the "Undead" will, which are more like living zombies than ghosts; Jack first appears to David with most of his skin still torn off from the Werewolf attack, and he just keeps decaying from there until his last appearance is... gah.
That little flap of skin that's just barely hanging on by a thread to Jack's neck during the scene in the hospital.
The nightmare sequences. Some screenshots should not be on rental video cases available to six-year-olds.
The first attack. Imagine you're walking with your friends in a pitch black field, you hear howls getting closer and closer. Shortly after that you hear the growls of the beast and youknowit's circling you.
The scene where one of David's victims is stalked through, and eventually killed on, the London Underground. We never, ever see the werewolf, but the guy's terrified reactions are more than enough.
There's actually one moment where we're given a very brief glimpse; a wide bird's eye POV of the man being carried up the escalator, and the werewolf emerges from the tunnels - the scene cuts to a different angle before we can fully register what we're looking at.
The massacre in Piccadilly Circus, where David bites the head off the intervening inspector in front of hundreds of terrified people. What follows next is a chaotic scene where dozens of terrified motorists try to avoid his monstrous form, crashing and mowing down several panicked pedestrians who are trying to escape.
The Nightmare Face that David makes in one of the dreams he has.(shown on the right)