There was a bit of discussion in the General thread on the subject, but I thought it might be worth its own thread. What movie, or movies, generally considered "great", did you not like.
Mine is 2001: A Space Odyssey. I watched it a decade ago, and absolutely hated it. I thought it was too long and slow, and I didn't get the ending.
I've heard people say that you really need to watch it multiple times to get it. But I honestly have no desire to do that.