I figure others will pick the well known and modern great movies, so I'll go back in the time vault a bit and pull out some lesser knowns.
Rules of the Game: 1939 french movie so many people today really don't li8ke it, but I was introduced to it in a film class in college and really enjoyed it
City of Lost Children: oddly enough another french film, this one 1997 and a little more well known. Very wierd, but fun.
Metropolis: 1927 american silent film about the working class. Don't see it with the Loverboy sound track if you can.
Abott and Costello meet Frankenstein: This is their best film, made in 1948. All the mosnters are played true to form and they got the classic actors to play them.
Duck Soap: 1933 movie and the best of the Marx brothers. Great comedy.