1) The Garbage Pail Kids Movie
Based on the popular series of gross-but-awesome trading cards for kids, this movie is mean-spirited, ugly to look at, and just plain difficult to watch. Many of the staff involved with making this have openly expressed regretting making it, and for good reason.
2) Masters of the Universe
Made to capitalize on the success of the He-Man cartoon and toy line, Masters of the Universe ran both over budget and behind schedule, and the result comes out looking like a mess. Not Dolph Lungren's worst role, but not exactly a winner either.
3) Jaws: The Revenge
Easily the worst of the Jaws movies, the fourth entry trades any possible terror for unintentional comedy. It basically implies that the shark is a serial killer targeting its victims. The film is so bad, Michael Caine has apparently never seen it despite performing in it.