Wow, you people haven't seen a lot of movies to see some really bad ones.... It seems your idea of a bad movie is Movie_1 > Sequel_1, therefore Sequel_1 = worst movie evar!
To me, for a movie to be absolutely horrible, I have to get in pain during the movie... mental anguish from actually sitting through the movie. I've seen a couple movies like this.
National Lampoon's Pledge This! was a very bad movie. It starred Paris Hilton for one and I don't know what it was, but I literally wanted to stab myself in the head many times during the movie.
The Comebacks was also a very, very bad movie. Nothing is worse than yet another spoof movie that is so lame and not funny that you wish you could stab the writers in the head. The only redeeming value of the movie was the hot Indian (dot not feather) chick and the coach's daughter

.
A couple dishonorable mentions are
Ultraviolet... the movie had horrid acting, a bad script and was absolutely boring to watch. Another is
The Ten. This movie presented these stories based on the ten commandments yet they weren't funny... there was no hawt lovin' scenes during "Thou Shall Not Covet Thy Neighbor's Wife" (very disappointing IMO)... in fact, that mini-story actually dealt with gay relationships in prison....
I can't really think of any more movies at this point as I think I try to put them out of my mind.