I thought it was one of the most boring movies I have ever seen. I could barely stay awake. Granted I saw it many years after it was released (probably in the mid 90's) but I had heard how good it was and finally got around to seeing it and thought it was so boring. I barely made it through it. In fact, I don't really remember much of it now so maybe I did fall asleep half way through.
ETA: Sorry forgot to list what I voted for......it was easily Jaws. Just a fantastic movie visually, story, suspense, blockbuster, and it still holds up extremely well.