Many people don't like Monty Python??? Almost everyone I know loves Monty Python.
Casablanca is mentioned in this thread, so, not sure how to respond to that, so I guess I'll let it go and say "to each their own".
For me, sometimes movies are bad because I expected more. Sometimes they're just bad. I'll take a crack at it.
Tarzan the Legend of Greystoke is high on the list. It's directed by the same man that did Chariots of Fire (Which a lot of people I know find to be tedious, but, I love it to death) with many of the same cast members. Ugh, just horrid. Was it mentioned that the director used Glenn Close's voice to dub over all of Andie Mcdowell's lines???? Bad stuff.
Battlefield Earth I had no expectations for, watched it on cable, but, was blown away by just how bad it was. Wow.
The 3rd Austin Powers movie really sucked, though, I bet some people will come to its defense, since I seem to be in the minority with this one. I loved the first one, the spoofing of the spy movie genre, etc. By the time he got to the 3rd one, it just felt like an extended fart joke to me. Fart jokes are funny, but not for 2 hours.
Rhinestone, with Sly Stallone, is painful to watch in many ways. To be fair, that's the only Stallone film I'll put on the list, cos I could add a few more.
Hoffa was fucking disappointing.
Catwoman. Watched the whole thing, though (Thank you Halle Bery)
Master of Disguise with Dana Carvey. Omg.
Ballistic: Ecks vs. Sever, just for the title.
Leonard Part VI
Ishtar (It really is as bad as the legendary hype about it being bad would indicate)
Staying Alive, the sequel to Saturday Night Fever. The last scene of that movie "I'm gonna strut" legitimately makes me feel uncomfortable, it's so bad.
Anything with Jamie Kennedy in it.
My ex made me watch Glitter. Glad we're broken up.
There is a movie called "THe 4th protocol", adapted from a good book of the same title. Horrid.
The New World bitterly disappointed me, since I love Malick's films. As usual, it was shot beautifully, but really was slow, and it takes a lot for me to feel that a film is slow.
I'll stop with Captain America, which few people know even exist. It really, truly, is the worst film I've ever watched. Supposedly, they're making another one in 2009.