Star Wars: Revenge of the Sith
This is objectively a horrible movie. I considered giving Attack of the Clones this honor, but ROTS deserves our scorn for how much better it should have been. Tom Stoppard attempted to save the film, and should have made it great, but Lucas sideswiped him so hard Stoppard insisted they remove his name from the marque.
Even James Earl Jones returning to voiceover the newly minted Vader was ruined by dialogue freshly shat out for this terrible, terrible film.