I have to say, the worst movie I've ever seen is "No Country for Old Men." The plot was all over the place, none of the characters were even remotely likeable, and it was just a bunch of gratuitous violence. No redeeming qualities whatsoever. I actually sat through the whole thing. But all the people who watched it with me hated it too.
I generally dislike movies that lack plot and revolve around idiotic men killing each other and everyone else. Don't get me wrong, I like action movies and some amount of violence, but there has to be a cool premise or interesting characters or at least some nifty reason for the killing sprees. It's one reason I avoid most horror movies, and I'll only watch them if there are cool monsters (but no vampires) or rampant cannibalism or other things I find interesting. I also like horror movies that don't take themselves seriously. The awesomest horror movie I ever saw was this film about a crazy "foreign" chef who gets a job at a sorority house and starts murdering the girls and serving them to each other. There's a lot of gratuitous nudity, horrible acting, and rather amusing death scenes. I forget what it's called, but it's awesome.
I also hated "Citizen Kane." I don't understand why a lot of uppity intellectuals praise it as the greatest movie of all time. Bullshit. The guy is a total douchebag, the movie isn't even interesting, and if I wasn't required to sit through the entire thing for a film class, I would have rented the Peter Griffin version and saved myself a couple hours of pure boredom.