This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / The Wicker Man
The islanders dancing about the Wicker Man, joyously singing "Summer is i'coming in" while Howie burns to death, howling in agony.
Edward Woodward said that in all his acting career, he had never been more terrified than when he was in that Wicker Man with it on fire. He shared many of the viewers' fears about that scene.
Also, there's something about the islanders wearing those animal masks that just isn't right. It doesn't help that the masks are very realistic. Or the way they just appear, and stare at Howie...
The shot of the Wicker Man totally engulfed in flames in the film's final seconds. By that point, you know that Howie is long dead.
If you look closely, you can see his burning corpse still leaning against the gate.
Howie dying all alone on the island, with no one on the mainland knowing what's happening to him.
Despite the Narmy performance of Nicolas Cage, getting a mask filled with angry bees stinging your face is a horrifying thought.
You can replace the angry bees with any other creatures that you have a phobia of, or an allergy to (spiders, snakes, scorpions, rats, worms, etc), and it's STILL horrifying. Compare to the infamous Ceti Eel scene from Star Trek II: The Wrath of Khan, or the rat torture scene at the climax of Nineteen Eighty Four.