There are a lot of high-impact horror movie scenes, but some represent some basic fears. I think about the ones that were pivotal for most viewers, that gave me them something they had not seen before, enough to make them gasp and to stick with them as something too scary to recover from fully. I am going to try to put them into "types" of scares.
Childhood Fear: "Poltergeist" (clown doll drags the kid under the bed)
Primal Fear: "Jaws" ("you're gonna need a bigger boat" moment when they realize they are in the water, vulnerable, and the shark is freaking enormous)
Curiosity Kills: "Jeepers Creepers" (the sibling go to the tube to look down it and find out what the creeper threw down there)
Family Can Turn On You: "The Shining" (the father goes mental and tries to kills his wife and son)
Being possessed: ("The Exorcist" final scene