Here's my Casablanca story. I saw it for the first time at college during a 40's film festival. During the first scene where Bogart is shown all the women in the theater gasped. It was an amazing, spontaneous moment. I had no idea until then how much raw sex appeal Bogart had.
I saw Citizen Cane in college. It worked for me because I was taking film classes and we dissected it.
I loved it more for the way it was shot (camera angles, scene composition, etc.).