Citizen Kane
I had trouble staying awake during the first part of this movie and, as I would find out later, I missed out on some important parts of the movie. It isn't that I found the first part boring, I was just tired and needed a bit of a nap. Eventually I woke myself up and started watching and geninuenly got interested in the film. As the final credits rolled I wasn't really sure how I felt, especially when the major plot device "rosebud" ended up not being the great revelation that I thought it would be. Still, I watched the commentary and that helped by pointing out a lot of things like camera work, etc that I would never have seen on my own...in addition I got to see the parts I missed.
I really enjoyed the movie, and would like to watch it again sometime...but I really wonder if I hadn't known that this movie was supposed to be a great classic would I have felt diffrently. Hard to tell since I can't find out. Easy to see why this one is consitantly number 1 in most lists.