Koyaanisqatsi (A Hopi Indian word for Life out of balance)...This is really a remarkable film. If you can ever catch this movie at a film fest in a theatre it helps..But watching on a big screen with a good sound system is the next best thing..I promise you this movie will mess with your senses. I haven't seen it on television in well over 10 years..But you can still rent it on DVD.
Other underappreciated movies:
The Stunt Man
Duel (Steven Spielberg's first film)
Blood Simple (the Coen Brothers first film)
Any David Mament film (House Of Games, The Spanish Prisoner, Glengary Glen Ross)