Synecdoche, New York is certainly status... Charlie Kaufman is a fukkin genius
Incendies is another good one... the end of that movie fukked up my whole night, didn't fall asleep until 7 in the morning
Vanilla Sky but I'm sure you've seen that, same with Identity and American Psycho
OP check out a 2009 film called "Moon". Something you probably haven't seen.
What is Incendies about?
No Stanley Kubrick movies have been mentioned?
2001: A Space Odyssey
A Clockwork Orange
Eyes Wide Shut
The Shining (there is a documentary coming out about all of the theories surrounding this movie called Room 237)
I know these aren't unknown movies... but should have been mentioned