It's always fun to read these threads to see how divergent peoples' tastes can be. Some of the most hated mentioned are my absolute favorites. Casablanca, Dr. Strangelove, Inglourious Basterds, to name a few.
Last Year at Marienbad, which someone mentioned way up thread is definitely my worst ever but I don't really count it as much because it was meant to be an arty film and whenever someone sets out to make a "film for the sake of art and anyway only intelligent high brow people will understand it so if you don't love it you must be stupid", it's pretty much going to be bad. I don't mind if they think I'm stupid.
The worst mainstream movie I've ever seen is Rachel Getting Married, which incomprehensibly got nominated for some Oscars. Yuck.
A close second was Blue Velvet. It was awful.
Some comments on other films that were mentioned:
Les Misérables - The play in the theatre was phenomenal. The movie was awful. It was long and boring.
Abraham Lincoln: Vampire Slayer - Look, you have to figure by the title it's not going to be the most serious intelligent movie ever. I thought it was very entertaining for what it was. Lots of action and fun stuff. Nothing earth shattering but I definitely enjoyed it.
Moulin Rouge - This is one of those films that people either love or hate. The first time I watched it, it took me about 30 minutes to get it. Once I did, I fell in love with it.
Anything by the Coen Brothers or Quentin Tarantino -- you either love their stuff or you don't. I do. But I completely understand why some don't. After Fargo came out and was so incredibly popular, I know people who went to see The Big Lebowsky thinking it was going to be like Fargo. The thing is, it was, in a way... it was very Coen Brothers -esque. But the thing about the Coens, all their movies are very different from each other, different genres, different kinds of plots, etc. They're all really good (IMO) but in completely different ways.
What About Bob? – I know I saw that movie, but for the life of me, I can’t remember a thing about it except that Richard Dreyfus was bothering someone else. I tend to block out bad movies unless they’re so bad I can’t block them out.