One of the very few "director's cut" editions that improved a film was Blade Runner, and that mostly by taking things out (Ford's awful voiceover, demanded by the studio and recorded basically under duress).
The director's cut for THE EXORCIST III reversed many of the studio-mandated additions to the film, including a B-plot with Nicol Williamson as an exorcist and Jason Miller reprising his role as Father Karras... and for my money, the studio cut's the better movie.
