Looking for movies where the directors cut completely changes the movie in a good way.

Fast Times at Ridgemont High's regular release is a pretty feel-good, meandering nostalgia movie with a few more serious moments. The extended edition turns it into a much deeper and more tumultuous film, and adds a sense of forward progression to the characters and story. This is fantastic.

Donnie Darko's extended cut destroys any trace of speculation or discussion, turning a haunting and memorable story into something more generic. It spells out literally everything that happens during the movie and overall subtracts from it, by most viewer's standards (myself included).