I'll name the movies that contain my favorite twists, but not what the twists are.
The Sixth Sense
The Usual Suspects
I normally don't like courtroom dramas, but I liked the twist at the end of Primal Fear. I think it was Edward Norton's first movie. He's such a cool, underrated actor to watch.
Also, I'm not sure if these two really count as a "twist ending"- both Mulholland Drive and Donnie Darko have endings that make you re-think what the whole movie was about and see it in different ways...
The revelation that Angela is a boy in "Sleepaway Camp".
The revelation that Angela is a boy in "Sleepaway Camp".
That last shot of her/him standing there naked with that crazy drop-jawed face... classic!