So, here's a no-brainer, right? One of the quintessential Horror films of all time, that just happens to be one of the quintessential Hitchcock films. The movie is sheer tense brilliance where the horror comes from where it should come from... the plot and the characters. Up until the final reveals of Norman and the Fruit Cellar, the paranioa and confusion is so overwhelming that you're kept on the edge of your seat, hoping... praying that whatever happens... happens and it doesn't hurt you.
The movie works as almost a selection of shorts with an interwoven plot. The first third dealing with Marion, the second works as an interlude following the private eye, and then the pot boiler ending section. Sure, plot wise they're all the same, but the tone is quite divurgent... I'd say the first section has more to do with the later films, like Marnie, the second a nod to the early Detective Noirs, and the final section is pure North by Northwest.... plus a rotting corpse.
Interestingly, like Marnie and even a bit in Rope, the only real downside to the film is the psycho-babble at the end. Hitchcock's obsession with putting a fine point on the psychological underpinnings of his heroes and villains is always too much on the nose for me, but, compared to most of the films of the era, it's still expertly executed.
The root of the modern horror movie is Psycho, and yet, we've gotten it so wrong since. Psycho will always remain a terrifying classic with no peer.