Star Trek: Nemesis
I've have enjoyed every single Star Trek film, even The Motion Picture. But Nemesis was an insult to anyone with an IQ higher than 100. There are so many plot holes and continuity errors, it's laughable. On top of that, the story was crap. The only good things about the film was its name and special effects.
Apart from Nemesis, I've never walked out of the cinema film before.