Define "best" however you will... critical acclaim, recommendation of friends, etc. What's the best movie you've never seen yet?
For me it would have to be "Raging Bull". It's DeNiro. It's Scorsese. I should be pre-programmed to love this movie. And yet, I've never seen it. What the hell? I even got so far as to rent it from That Chain That Supposedly Doesn't Have Late Fees But Really Does, and unfortunately, work got busy and I had to send it back unwatched.