I went with the Obi-Wan/Vader Mustafar duel. *The* most anticipated cinematic experience of my life, and they nailed it. The best choreography, acting, visuals, and tension of all the duels in the saga, and the music is right up there with the very end of the Luke/Vader fight in ROTJ (with ROTJ probably edging it out a little.)
Ewan McGregor's pitch perfect delivery of "I have failed you Anakin, I have failed you," gives an emotional punch that actually exceeds the visual splendor of the battle itself. It was everything I could have hoped for.
Second place would be Luke vs. Vader in Cloud City. One of the defining moments of my childhood, and I know it will have a nostalgic resonance for the rest of my life (in addition to still having great tension and drama, even today.)
And extra kudos for the Yoda/Palpatine fight turning all those dull senate chamber sequences into one big build up for a climactic action sequence. *Never* saw that one coming! Bravo.