SeventhDegree listed a number of sequels that should never have been made that qualify as candidates for the worst movies ever made. I'd like to add some:
The Crow: City of Angels (a no-budget, no-name rip off of the previous film)
The Crow: Wicked Prayer (an even less-budget, some-names flaming turd of a film)
Highlander II: The Quickening (there should have been only one!; undid much of the backstory of the first movie, with unbelievably bad special effects and hammy acting)
Highlander: The Source (a really bad episode of the TV series with an even smaller budget)
Blues Brothers 2000 (unless you like the music so much that you find the film secondary)
All of the Police Academy sequels, especially the post-Steve Guttenberg ones
The third Crow and Highlander films, although bad and unnecessary, are not nearly as bad as some of the others in their respective series. I thought Eric Mabius was an improvement over Vincent Perez. Highlander: The Final Dimension, was a logical continuation of the first film, forgetting the second film ever happened. Against my better judgment I kind of like the fourth film, Highlander: Endgame, at least as a guilty pleasure. The Source has no redeeming qualities whatsoever, unless people find the aligning planets special effect hilarious.