A lot of people have pointed to the shower scene in Psycho, but that was never for me, mainly because I was aware enough of pop culture to know it was coming, even the first time I saw the film. However, I very much liked the later scene when a fellow (cop? detective? something like that), was slowly coming up the stairs of the Bates House, and there is this creepy overhead shot where you can see both him on the stairs and the upper landing he is approaching. When the killer strikes, it isn’t a sudden, violent, stab, coming out of nowhere, like in most other horror films. You see the killer run across the landing, knife in hand, and you’re aware of the coming attack just before the victim has any idea of what’s coming. There’s a breif second where you just want to shout “look out!” but the moment passes too quickly for him to hear you and react, and the deed is done.
Also: big ups for the tension in the scene from “Rear Window” when Jimmy Stuart is watching his lady friend snooping around the killer’s apartment, and you see him come home through a different window. Hitchcock was the master alright.