Alice killing Mrs. Voorhees with a slow motion decapitation.
The end where after Alice kills Pamela Voorhees, she sails off into the lake on a canoe, and when everything seems peaceful and serene, the decaying child zombie of Jason jumps out of the water and drags her under. She then wakes up in a mental hospital and says "he's still there". Despite being labelled horror, most horror movies don't tend to scare people who just see them as just killers and jump scares. This scene shows that it isn't those things: it is horror. This is one of the most horrifying scenes in movie history and to some, the best moment in the entire Friday the 13th franchise.
The movie does an excellent job of concealing the killer, making the revelation that it's been a middle-aged woman the whole time genuinely shocking.