This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / House on Haunted Hill (1999)
The scene where Melissa finds herself videotaping a ghostly vivisection in an empty room, and then senses something behind her. She turns, sees a shadowy figure peer around a corner way down the hallway, and in an eyeblink it's right there in her face, teeth, blood and all.
The worst part prior to the creepy-jittery ghost suddenly attacking her? She only sees the vivisection on camera. The room seems empty and abandoned, but through the screen of the camera, the doctors and patient can be clearly seen... And then the doctors turn to look at her.
The scene where Steven Price discovers that another character's face and the insides of his head are missing (this discovery is accompanied by Ominous Latin Chanting, of course). To make matters worse, he sees the unnaturally twitchy ghost doctor taunting him with his large bloodied knife on surveillance footage. See the scene here.
The nightmare sequence in which Steven Price is in the "saturation chamber." First, his head is tightly wrapped and dunked in water by twitching creepy ghosts. Then, he sees a beautiful young woman submerged in water who starts bleeding from the mouth before revealing her Nightmare Face (which consists of smooth-ish skin where her eyes and nose should be and a GIANT gaping mouth◊) while screaming at him. Finally, he is exposed to his wife holding his (still alive!) severed head while laughing evilly. See the creepy scene here (NSFW because of brief nudity, also seizure warning).
Steven Price's wife rotting to nothing by "the Evil."
The final shot at the end of the credits, showing the characters devoured by the "Evil" in a nightmarish scene right out of Vanacuts depraved "Home movies", suggesting they're still trapped in an endless hell within the Asylum. Even the opening credits are creepy.
Nobody ever seems to mention when Sara has her own encounter with a spirit in the basement. To basically explain: Sara looks for Eddie after she walked away from him when he figured out that she was actually an assistant named Sara Wolfe and not Jennifer Jenzen. At first, she gets annoyed, thinking he's pranking or making fun of her. Then she finds him jumping drowning in a tank filled with what looks like blood. When she begins to panic and try to help him out, she keeps calling his name. Until she ends up hearing the real Eddie in front of her. So who is in the tank? Cue Sara almost being pulled in by the thing and Eddie realizing she's being attacked.