This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Trick 'r Treat
While still a horror film, most agree that the film is not a scare fest. However, almost everyone agrees that the ending to the school bus story is the most chilling moment in the film.
While we don't see anything of what's happening down below as Rhonda reaches the top of the cliff in the lift, the sounds of the kids screaming as they're torn apart - and then the sounds of whatever comes next - are extremely creepy.
Rhonda's face expression as she hears everyone be killed makes it worse, sort if hinting she's either undergone a Sanity Slippage or become flat out psychotic.
The story of the the Halloween School Bus Massacre that precedes the above, told in flashback. Thirty years ago, there were a group of eight mentally challenged children in the town, who were sent every day by bus to a school in a different town. One day, their parents, tired of taking care of them, paid the bus driver to kill them. The driver chained the children to the seats and drove to a lake in an abandoned rock quarry, intending to drown them. One of the children managed to free himself but accidentally drove the bus into the lake, drowning everyone...except for the driver, who escaped and is alive and well.
"Wrong way...wrong way...wanna go home...wanna go home...home!"