I'm a sucker for the twist or the big reveal when something hasn't been what it seems :
* When Tyler explains to Jack the true nature of their friendship in Fight Club.
* When Patrick Bateman confronts his lawyer Howard in the wake of his confession in American Psycho.
* When we see at the end of Memento the beginning of the chain of events for Leonard that climax at the beginning of the film.
* When Ed discovers the real cause of the murders in Identity.
* When Teddy learns the truth about Rachel Solando and Andrew Laeddis in Shutter Island.
My all-time favourite scene however, may be the interrogation of Catherine Tramell in Basic Instinct. No, not because of the flashing, but because of Stone's awesome portrayal of what we need to know about Tramell - cool, composed, dry, dangerous, intelligent, sardonic, seductive, psychotic. The end of the scene where Nick is watching the monitor of her taking the lie detector and she looks right into the camera at him is awesome. I wish more people took it seriously as a thriller rather than a film to wank over, it's excellent.