I first watched It's a Wonderful Life one night shortly before Christmas break while I was at A&M. Caught the very beginning when the galaxies were talking to each other. That seemed so unusual that it grabbed me and little did I know I was watching the movie featured on a TV screen in Gremlins.
About a year later Lawrence of Arabia was released theatrically, so I was blessed to get to see that on the big screen. Simply a great movie on all fronts.
I first saw Patton in about 1993. Another damn near perfect movie. My mom, who knew of my life long interest in WWII, was surprised I had never seen it.
Other titles that impressed the heck out of me include Judgement at Nuremberg, All the President's Men, The Bridge on the River Kwai, The Caine Mutiny, Fail-Safe, The Bedford Incident, Dial M For Murder, Vertigo, Rear Window, North By Northwest, The Flight of the Phoenix, The Godfather (1 and 2), The Manchurian Candidate, and many John Wayne movies like The Searchers and In Harm's Way.
Raging Bull bored me to tears.
There are some titles that I still need to watch, like A Clockwork Orange, Taxi Driver, and The Maltese Falcon.