I can't disagree with any of the films posted so far, so I will just add a few that I haven't noticed anyone list yet (I may have missed them if they were)
2001: A Space Odyssey
Memento
Donnie Darko
Dark City
Inglorious Bastards (OK, this one hasn't even been made yet, but how can a WWII Tarantino film not be a "must-see"?