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 and seemingly endless field, you hear howls getting closer and closer. Shortly after that you hear the growls of the beast from the darkness that surrounds you and youknowit's circling you.
When the pub-goers are asked about the crucifix on the wall, they become very hostile and the boys decide to leave. However, they insist that the boys avoid the moors, stay on the road, and beware the moon.
After the boys leave, we see that the pub-goers, apparently regulars, have barricaded the doors of the pub and drawn their weapons. The barkeep insists they shouldn't have let them leave, but is rebuffed. Then we hear the most hellish, demonic howl in the distance, and all the pub-goers look up at it. The howl continues. The barkeep begins losing her composure and begs the pub-goers to go after the boys. The others refuse, too afraid to leave. One says "it's in God's hands now." The pub doesn't get much screen-time, but damned if this scene didn't make you afraid of the unknown.
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)
The theatrical poster works thanks to, again, fear of the unknown. It depicts Jack and David looking off into the distance uneasily, with the full moon overhead. The tagline reads, "From the director of Animal House... a different kind of animal." The poster really puts you in the moment as if you're the unfortunate soul looking off in the distance as you hear the howl of the approaching werewolf.
Towards the end of Jack's first visit, he gets serious when it's clear David isn't buying what he's saying, and tells David that "the forces of darkness" and the supernatural are Real After All and it's why Jack is stuck as the undead and David will become a werewolf. It's terrifying enough to have confirmation that there really are evil forces out there.