Psycho
My parents always liked to tell the story of going to see Psycho in the theater. Apparently the scene where the detective met his demise was so shocking that my father got a nosebleed. That's pretty shocking, right?
But even as a teenager discovering Hitchcock's films for myself, I found Psycho rather disappointing. I already knew of Janet Leigh's fate, since the shower scene is one of the most famous in film history, so no shocker there. There were a couple of good, jarring moments and Anthony Perkins puts in a tremendously good performance as Norman Bates, but I always felt like I was missing out, seeing the film about 30 years too late. I knew too much. I'd seen loads of modern suspense and horror influenced by it.
I also imagine that it was fun to get caught up in the marketing of Psycho before seeing the film. Hitchcock requested that theaters not allow anyone late entrance into the film and begged filmgoers not to reveal the ending. Not exactly subtle, but he was treading fairly new ground at the time. (I just read that he borrowed this marketing from that surrounding the release of the French film Les Diaboliques, which also had a moment or two that made me jump out of my skin.)