Man, went on vacation last week and just saw this thread now. So many movies to choose from, I don't know if I could pick only one.
Dazed and Confused, Rear Window, Dial M for Murder, A Clockwork Orange, Matrix, The Dark Knight, Casablanca, Spirited Away, M, Pulp Fiction are all very high on my list.
I think my favorite would be Lord of the Rings: Fellowship of the Ring. I like all three just as well, but being a teenager and seeing the first movie got me into it so much that it's tough to beat. As a teenager, those movies blew everything else out of the water.