7. Superman II
Richard Donner did such a superb job with the original Superman that it's amazing to think anyone bothered to challenge him on developing a sequel, but alas, producers chucked him off the set with 75% of the film finished, and appointed the seriously inferior Richard Lester to finish up production on the film. Lester not only re-shot several scenes, but added in some unsavoury slapstick just to make the whole thing even more embarrassing. Lester went on to make the abominable Superman III, proof that any watchability the original cut has is surely down down to Donner. Thankfully, Donner was eventually allowed to release his own cut of the film back in 2006, and though by its nature it's imperfect - test footage had to be used to fill some gaps - it is vastly superior to Lester's mangled take. Tragically, most remember Superman II as a largely inferior sequel precisely because of what the studio did to poor Richard Donner.