This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Dark Water
The film contains many such moments, but the biggest is the climactic scene in which Yoshimi, clutching a half-drowned Ikuko to her body, desperately tries to escape in the elevator, which, despite all her efforts, absolutely does not work, and shedloads of water begins to fill the elevator. Suddenly, the door to their apartment sloooowwwly begins to creak open. Yoshimi, still desperately trying to get the elevator to work and half-paralysed with terror, can only watch as the door eventually opens and out emerges the figure of... Ikuko? But wait, if that's Ikuko, then who is in the elevator with - AAAAAARRRGGGHHHOHGODDDDD.
What makes the film work so well is the atmosphere: As the film goes on, you realize there is no one in the entire apartment complex. It's just Yoshimi and Ikuko.