There are three types of bad movies:
1. Cult classics. Different, quirky - appealing to some, bad to others. Rocky Horror, Repo Man, perhaps Tank Girl...
2. B-fest/MST3k bad. - They are bad but unintentionally quite funny. Any Ed Wood movie, Godzilla 2000, Cool as Ice, No Holds Barred, Glitter, Pearl Harbor (only for the history buff, just plain bad otherwise)... For TV you can even add celebrity boxing to the list.
http://www.b-fest.com/ 3. Bad. No redeeming qualities. Any Tom Green movie. The Sweetest Thing (ARRGH!) The most recent one that jumps to mind would be Adaptation (if I wanted to watch self-serving, self-important performance art I would go downtown and pay to watch some tool shoot **** all over the audience while he bleeds on stage).
Charon