This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Inception
The implied horror of Cobb's alluded-to past with Mal; even explained, it is still quite creepy.
The scene with the elevator that leads to his subconscious.
Mal's "real world" death. Cobb goes to their anniversary hotel room, dressed up and with flowers. He comes in and sees the room trashed. As he comes to the balcony, he sees Mal clinging to the building across the street, ready to jump. Cobb tries to talk her down, but she mentions that she's told people she's fearful of him. He turns around, surveys the room, and realizes he's about to be framed for murdering his wife. Then she jumps.
It is implied that the projections in Saito's dream ripped Nash apart in the opening dream sequence.
The basement full of people in Yusuf's basement, implied to be dream addicts who come to dream for hours upon hours every day because they can't handle the real world any more.