There are so many classic Hitchcock films it's hard to chose just one. Psycho is a prime example -- although today everyone knows the ending so it really kills the whole thing, but imagine NOT knowing what was going to happen and to see the star of the film, not only steal money, but die a brutal death approximately 20 minutes into the film. (And you never actually see her getting stabbed. It's all so well edited that the knife never touches).
Or how about Notorious? Even though I know how the story goes, I'm always tense that Ingrid Bergman and Cary Grant will get caught by Claude Rains? Or that seductive scene with Grace Kelly & Cary Grant in To Catch A Thief (you didn't really think she was talking about diamonds, did you?). Or how creepy Mrs. Danvers is in Rebecca (Joan Fontaine doesn't even have a character name in the film! It's all about the dead woman! Creepy). Or Strangers On A Train -- you kill for me, I'll kill for you! How many times has that been used since? Or Rear Window -- what people REALLY did before tv came into vogue (and again, even though I know the story I squirm when Grace Kelly enters Raymond Burr's apartment and all Jimmy Stewart can do is watch!). Or Vertigo -- is Kim Novak the same woman or not? Or his last film, Family Plot, which is funny and full of wonderful twists and turns -- is Barbara Harris really psychic?
Can never pass up a Hitchcock film. (And don't forget wife Alma was often the driving force behind the man, working on the films behind the scenes too)