For me it's probably Memento. The plot synopsis is basically this: a young man's wife is murdered and during the incident, he loses his ability to form long term memories. He forgets everything after a few moments, so, in a sense, he's constantly "thinking backwards" to try and sort out what's going on. He has tattoos all over his body to remind him of clues of who killed his wife and plans to get revenge.
The movie moves backwards in time throughout the color scenes and in black and white scenes moves chronologically. Makes it a bit hard to follow. Was a pretty good movie though.
Also, I thought Eternal Sunshine of the Spotless Mind, Requiem for a Dream, A Clockwork Orange, and Waking Life were all odd- but in good ways.