1960. dir. Alfred Hitchcock, starring Anthony Perkins, Janet Leigh, Martin Balsam.
Ahh, yeah, so, spoiler warning.
What appears to be an interesting morality tale about a young woman (Janet Leigh) who steals a large sum of money from her workplace and skips town gets utterly derailed a third of the way through when she gets randomly stabbed to death by the guy running the hotel she stays at. He's dressing up as his mother - who died ten years ago - because he's crazy. Don't worry, a psychologist explains the whole thing at the end for us.
This movie fails my "twist ending" test, in that I don't think the movie really works if you already know how it ends. Movies that pass the test include: Empire Strikes Back, The Sixth Sense, and Memento, which I think are actually improved when you know what's coming. I don't think this holds up without the shock value.
Position on the list: 22
Wait, really? 22? Ooooooo-ver-rated where the wind comes sweeping down the plain