The Sixth Sense-- an instant classic. The ending fooled me even though a friend had told me all about it right before the movie-- I had become so engrossed in the movie I had forgotten about it. The whole audience was stunned-- I could hear gasps from every corner of the theater as the realization of what had really been happening throughout the whole movie finally sunk in. Even after several re-watches, the movie itself before the suprise ending is powerful and emotional on its own-- and the ending, while not packing the same punch it first did at first sight, is still a simultanously heartwarming and hearbreaking finale.
Another classic movie with a twist I haven't seen mentioned yet is The Sting.
Another classic movie with a twist I haven't seen mentioned yet is The Sting.