This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / A Tale of Two Sisters
The immediate aftermath of Su-mi's first nightmare - as she starts to calm down, she hears a noise, just barely audible, coming from the other side of the room. She looks, and, after a few seconds, spots a pale, long-haired woman slowly crawling on all fours across the floor, causing her to gasp in terror and start to hyperventilate. This, of course, attracts the attention of the ghost - who proceeds to suddenly stand up in a split second before advancing on Su-mi, who is now too terrified to move. The utterly terrifying ambient score does not help.
The first nighttime scene in Su-yeon's bedroom. Her door slowly creaks open of its own accord... she shrinks back in fear... and after a moment of tenseness, a hand suddenly grips around the edge of the door. Gaaaah.
The scene where the wife of the girls' uncle starts having a fit. It is frightening enough the first time around, but, on a re-watch, if one believes the theory that she was being possessed, the scene takes on a very different light.