Well, the most improvement award goes to the Daredevil Director's Cut. But since Daredevil was still a bad movie...
I would say it is a toss up between Superman II: The Donner Cut (which I voted for) and Watchmen: The Director's Cut (never the "Ultimate Cut" though). But whereas the longer Watchmen cut seemed mostly to indulge my comic book fanboy tendencies of wanting to see more of the book on the screen while Superman II actually was improved (save for Donner erasing the "care to step outside, general" line), I voted Superman II.
X-Men: DOFP - The Rogue Cut is great, but it is just for fans again. The theatrical version is a better, leaner, and harder hitting film.
Most of the rest actually seem to take away for me. Spider-Man 2.1 had a few nice character beats for Harry or MJ, but stuff like JJJ wearing the costume? No thanks.