my answer is:
The Empire Stirkes Back. The main reason is that it is as close to a movie where the bad guys win as main stream Hollywood has ever produced. The overall darker feel was also a welcome change.
Other great things about it include: Wedge takes down an AT-AT, which Farm Boy couldn't do in his speader; The debut of Yoda and Boba Fett, two truly cool characters; and, of course, the Imperial March.