The Seventh Seal: This movie is often dismissed because it is Swedish movie that is subtitled but actually very famous and scenes are often parodied in comedies such as the muppets and Bill and Ted's bogus journey. This movie filmed in 1957 stars one of my favorite actors Max Von Sydow as a soldier coming back from the medieval crusades. He is approached by Death and through an ongoing game of chess yearns to find out if God is real. I really like it because the acting and cinematography is great and the story of not only this man's journey to find God but the other people it portrays are fantastic.
Metropolis: I avoided this movie for a long time but did not regret watching it. Metropolis is a silent movie from 1927 (hence the avoidance) which portrays a dystopian future society where the poor people work in shifts to keep all of the machinery for the rich people's luxurious life working. A man from a rich family decides he wants to find out the source of his luxury and goes to work. After doing so he helps with a revolt against the rich. I love this movie because both the plot and the special effects were ahead of it's time. It is long, but the fact that you do have to read it keeps you entertained.
Speak: This movie is often avoided 1. it has Kristen Stewart, 2. it involves rape. The fact is Kristen Stewart is a good actress, when she's not in Twilight. and I hate to say it, rape, especially teenage date rape, happens a lot more than people think and you don't know it until somebody comes out with it.
Mission Impossible Ghost Protocol: I can't tell you how many times people have told me oh yeah I saw that movie and then they will describe the plot of the 2nd or 3rd Mission Impossible movies (which in my opinion are garbage). This movie had a great plot with a ton of cool spy technology. and somehow I was able to look past Tom Cruise as being well, Tom Cruise.
PCU: It came out in the early 90's and I don't think it got enough reception. This movie is about a Pre-Freshman who visits a college in order to find out if it's the right fit for him. he ends up with a group of burnouts in a former frat house where he gets schooled in P.C.U. Politicaly Correct University. It's really funny. My favorite is when they dump the raw meat on the vegan protesters 
Clerks: Often avoided because of it's modern B&W low budget film making, Clerks is full of hilarious but crude 1 liners, hi jinx, and elements that make a great comedy. I remember I had seen almost every Kevin Smith movie with Jay and Silent Bob before I finally decided to watch this movie. The sequal is still just as funny. 