I'm going to have to go with the Return of the King. It was about 3 hours long, but inthe theater it felt longer because I had to go to the bathroom about halfway through it. The problem was, I couldn't get up to leave, because I felt like I might miss something important. I don't think my bladder will ever forgive me for that. ^_^
So far, it was The Dark Knight. I got through half of it and the Joker was caught, so I though it was ending soon. Man, was I wrong...It was more than 2hrs, but I don't have time to watch longer movies than this. I prefer to watch them all the way through.