The Deer Hunter. I first saw it when I was in my early teens and didn't care about the guys, the wedding, hunting, or any of the opening activities. I was not looking at the characters nor how they were reacting nor noticing themes.

I watched it again in my 40s and thought it was really good. The way the men communicate (and don't), the things that failed to stay the same for the characters, the loss, and everything -- it all felt deeply poignant. I still thought the movie was a bit too slow, but I finally 'got' why people respected it.

I had a similar experience with Lawernce of Arabia, but I liked that one as a youth and on re-watch was amazed at everything that went over my head, which made the adult viewing much better and cemented it in my mind as one of the best movies of all time. Another of the best is The Man Who Would Be King, but I've always abjectly adored that one.