There are quite a few films that are extremely popular that I absolutely hate. People have been giving me grief about it for years but I cant stand Casablanca. It embodies everything I hate about Hollywood, the so-called Golden Age of cinema, and the entire film industry in general.
That being said, Id have to give the award for my least favorite films (at least that have been made in my lifetime) to the Lord of the Rings trilogy. I think almost any film can be enjoyable on some level (whether it be something that genuinely impresses you or is so bad that its good). Very rarely does a film come along that I cant at least watch to laugh at and make fun of (like Manos, Troll 2, or Laserblast). LOTR, however, fits into that category. I can watch maybe ten minutes before turning it off.
There was also an adaptation of Ulysses made in the late 60s that is just awful. I knew it would be bad but was curious as to how they would translate that material. The result was, of course, failure.
http://www.imdb.com/title/tt0062414/