First, there are the truly underrated movies, movies that while mainstream critics universally call bad. A prime example is Pootie Tang. This should actually be a category of good movies rather than bad, but I'm listing it here because I can.
Then there are, of course, overhyped pieces of junk like Transformers, Men In Black, Titan A.E., Star Wars: Episode I, that probably aren't the worst movies of all time or even close, but just irritate me because of sheer popularity...and are lousy.
Then there are those indie films or critically acclaimed films that make absolutely no sense and thus I hate for the absolute lack of sense they make, and how the craft and care can't make up for the ball of confusion I've been twisted in after watching it. Beloved and Satyricon fall here.
Then there are failing sequels like Spider-Man 3, X-Men: The Last Stand, Men in Black II, Shrek the Third, and Batman and Robin.
Then there are some movies that while not of the greatest quality and are critically flawed in several areas, contain just enough goodness to tick you off, making it seem worse than what it was. Watchmen, Underworld, Ghost Rider, Battlefield Earth and Johnny Mnemomic.
Then there are some movies that would be decent if they didn't end horribly. John Carpenter's Ghosts of Mars comes to mind.
Then there are some movies that really are on the low end for most of their running times, but end nicely enough to elevate the mateial beyond the bottom of the barrel. Aliens vs. Predator is a fine example of this.
Then there are some movies that are low-budget or plain incompetent (sometimes intentionally so) that are so bad that they're entertaining. Dracula 3000 and Bones come to mind here.
Then there are the really bottom of the barrel movies, movies so bad that the reviews did not adequately prepare me for the EPIC FAIL of these movies and failed to have me do anything by wonder why I watched them. Things like Wild Wild West, Norbit, Cursed, Belly, Hulk, Deliver Us From Eva, Inspector Gadget, The Breed (the dog movie, not the Adrian Paul one)...and now Twilight. This is the level of badness that makes me want my money back.
And then there's Tank Girl, a movie I consider the worst of all time. There have been movies that come close, and there are some days in which I consider Highlander: The Source to actually be worse, but nine times out of ten, I'll say Tank Girl, because at least I could understand Highlander: The Source.
Edit: I've since seen Dragonball: Evolution. It's so bad it's entertaining, but not as thoroughly terrible that it's not watchable. So it's on the Dracula 3000 level, which is miles better than I expected, sadly.