The first of these is Gigli. The movie, which came out in 2003, should have been a hit, as it featured two of the biggest stars of that time: Jennifer Lopez and Ben Affleck. It turns out that two famous people don't necessarily make a good movie. The movie flopped miserably and is really only remembered as one of the worst movies of all time.
Mariah Carey came out with her movie, Glitter, which was another total failure. It felt like a movie that only a little girl should enjoy, and yet we're not sure if anyone, even the little girls gave it a good review. It's a gaudier, glitterier version of something like Mariah Carey's life, and it's painful to sit through.
Not many people actually saw the animated sequel of the Titanic, yet it was created and ranks among IMDB's worst movies of the decade. It's a rip off of the original Titanic with the roles reversed and the character animated. The two main characters look suspiciously like Jack and Rose, and it's a shock that there weren't dozens of lawsuits filed.
Honourable mentions on this list go to Battlefield Earth (in which everyone looks like an alien version of Whoopi Goldberg), The Love Guru (racist and irreverent to the max), and Catwoman (Halle Berry wasn't even enough to save this movie). They're not so bad they're good, but they're so bad they're torture to watch. There are many more, but these are the ones to avoid at all costs.