Possibly, Alfred Hitchcock's most famous film. The Master of Suspense's Psycho is so chock full of twists and turns that it's hard to pin down just one famous twist, but we have to go with the final pair for the sake of mere iconography. After a long and chilling thrill ride (wherein our lead protagonist is murdered halfway through the film; unheard of at the time, and still rare today), Vera Miles spins that chair around in Norman Bates' cellar - only to discover that Norman's mother has been dead all along. And before we can recover from the shock of that development, in bursts Norman himself, dressed to the nines in his mother's clothes and wig and brandishing a huge knife, ready for the kill. It's a horrifying twist that expands the character in a myriad of disturbing ways.