I was on a flight once and had to endure "What a girl wants"......mind numbingly bad i had to bite my fist the whole way through.....so bad in fact i got copies for five friends for that christmas....its like chinese water torture but with poor dialog...each moronic plot move another drip on the forehead
Twilight (can I say all of them this time?)
The Star Trek Movie (not a bad movie, necessarily, but the main character is a chauvinistic pig)
Burn After Reading (Incomprehensible)
I have been warned off the subsequent Matrixes by my brother, though I've never seen them
SPIDERMAN 3!!! (I didn't much like 1 and 2, but 3 was the worst by far)
I saw one of a mother who goes crazy with PPD; and ends up burying her baby, convinced she was a doll. It isn't worth googling.
Star Wars Holiday Special. Hands down.
If we are talking about the book to movie conversion the worst I have ever seen is Ella Enchanted. The book was beyond amazing and the movie was just awful. It actually made me hate Anne Hathaway for a while for agreeing to be in that travesty of a movie.
Plan 9 From Outer Space is notoriously bad. Queen of the Damned is horrible if you've ever read the book but it's got a killer soundtrack, Ghost Rider was pretty horrible not one of Nic Cage's greatest achievements. I dunno there are so many bad movies I don't even know where to start really lol.
Elvira, Mistress of the Dark
Bug, Push, the entire twilight movies. (horrible horrible acting, don't even bother trying to tell me otherwise.), glitter, gigli, battlefield earth (altho visualy it was nice..but storywise not so much), wild wild west, adventures of pluto nash, The League Of Extraordinary Gentlemen, Alone in the Dark, Sphere, The Master Of Disguise, The Tuxedo, Christmas With The Kranks, Catwoman, Boat trip, babylon A.D., Envy, In the name of the king: A dongeon siege tale, rollerball (2002) and finaly Kickin it old skool. These are not in a particular order...just stuff i could remember off the top of my head that in my opinion sucked pretty bad. I could probably find more but yeah.
2012 - "The neutrinos are mutating" Need I say more?
or maybe Transformers: revenge of the fallen and Transformers: Dark of the moon, they both managed to hurt my childhood just a little bit.
I especially like the part where the lead character out-runs the Yellowstone super-volcano eruption. Very classy.