This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / The Ring
The 1998 film of The Ring. It's not the sort of film that jumps out and scares you. No, this is Fridge Horror terror. This is terror that comes creeping up on you only after you finished the film and have walked away. Then you start THINKING. To make matters worse (or better if that's your bent) this is quite possibly the only movie ever made that actually gets MORE terrifying simply by watching a 4th generation VHS copy of it.
The American remake has two jump scenes with the corpse and the horrible electronic shriek when Samara's victims are revealed.
The "scared-to-death-face" can also freak one out.
The original's simple twitchy movement by Sadako. Ring 0: Birthday has a movement-moment: when most of the victims are dead, the final women, Akiko and Etsuko, take shelter in the old clinic; Sadako is some distance behind them at the time they attempt to hide in the building. Akiko then runs back to barricade the entrance… holy shit, Sadako's suddenly appeared right in the doorway. A terrified Akiko then runs around the corner back to Etsuko and they both cower, too frightened to move. Nothing happens for several seconds… then Sadako suddenly appears around the corner, her limbs contorting in a horrifying manner, complete with bone-crunching sounds because the theater troupe HAD broken every bone in her body when they killed her. This is Sadako re-arranging herself and moving on sheer psychic power alone. Which is probably even worse.
The bit just before where the Akiko and Etsuko are running through the forest and you can SEE Sadako just standing in the background, watching, waiting. One fall later, they get back up and she's gone. Sets your teeth on edge.
A truly disturbing moment in the 1998 film arrived in the Deliberately Monochrome flashback to Sadako's mother giving a demonstration of her psychic abilities. The reporters present begin to shout "Fraud!", and all of a sudden, one of them just drops dead, wearing the same twisted face as Sadako's present-day victims. We cut to the present for a moment as the protagonists react to this information, and then, without warning, we cut back to the flashback. Sadako runs out from behind the corner — our first real glimpse of the Stringy-Haired Ghost Girl — almost scuttling, like an insect. A subtle moment in the overall scheme of the movie, but truly effective.
The original tape was supposedly created when someone taped a mysterious broadcast late at night from an unlisted TV station.
The line from Aidan in the 2002 film: "You weren't supposed to help her!".
When in The Ring TwoAidan's mother fixes up a sleeping-pills-peanut-butter-and-jelly sandwich as she prepares to drown him.
The part in the 2002 version when the heroine is climbing out of the well while Samara is creeping UP the wall below her. It's the legs. The way they suddenly step in front in a logically spine breaking way (that and the constant slow speed of the "walk").
The page image from the 2002 version. The Reveal of Samara's face is pretty damn terrifying. It's not so much the rotted, undead nature of the face, it's the amount of raw, seething hatred in her expression.
In fact, the second time Samara delivers the lines with a different context, as revenge than apology:
Dr. Scott: You don't want to hurt anyone.
Samara:(angry) But I do, and I'm sorry. It won't stop.
In the 1998 version, you only see Sadako's eye glaring down with intent. What's worse, is you can't stop staring at it.
For the remake, the video itself. Even when each of the surreal shots were given context.
Then when Rachel first looks at the photographs of the teens post-videotape watching, with the screech of the monorail above adding a spine-tingling chill.
The Ring 2. The "why won't this delete?" scene is widely considered the single most terrifying moment in the whole film (in addition with Sadako's well ascent during the climax). Okazaki really shouldn't have chickened out of watching and passing on the tape.
The tape itself is just horrific. It's a collage of short, disturbing scenes that seem to be non-sequiturs, but make a terrible sort of sense once you know the backstory.
The first novel has a few eerie moments aside from the main plot. When Asakawa arrives at South Hakone Pacific Land, he does so on a road that is unusually narrow for the entrance to a resort, which then suddenly widens as he gets closer. He stops at the main building to ask for directions, and there are plenty of people there, even though it's late at night on a week day. There's something about the small part of the novel that's just so darn creepy.
The mysterious voice on Sadako's tape (theorised by some to be the voice of "the Towel Man") in the 1998 version, which can only be heard clearly when the tape is played in slow-motion, is incredibly eerie and creepy. "If you keep on doing Shomon, Bokon will come." (Incorrectly translated in some versions as "Frolic in brine, goblins be thine" although the basic meaning is the same.)
The horse suicide in the 2002 remake forced some people to turn the film off or leave the theater.
The subway scene in Sadako 2 has an entire train being haunted and slaughtered by Sadako instantly. The aftermath is even more scary as it looks like how an actual disaster might happen - people in a panic or shock, paramedics running in with stretchers, etc.