It's a close tie between Empire and Revenge. I love Empire because of the darker turn it took from A New Hope (although I ADORE that one) and Revenge holds sentimental value for me since it was the first one I saw in cinema, and I fell in love with the characters thanks to Stover's novelisation prior to watching it. If I only had to choose one, it'd be Empire - just because Vader is so badass. Also, Boba Fett! And Lando! I love the OT.