ESB (Vader vs Luke)
The reason this is the best duel, IMO, is because this is the only duel that combines great intensity, great drama, great action, and great surroundings. All of the other duels have good areas (TPM has a great action but zero drama, ROTJ had good drama, but little action because it is such a short duel. ANH has great dialogue but lame action, ROTS starts off with a bang on Mustafar but loses steam once they start lava surfing as it becomes a video game at that point)
If you watch the ESB duel, it is the only one where you really understand what Luke is going through, as Kershner did a great job of showing the duel from LUKES POV. The Duel builds and builds as Vader is kicking Lukes as physically and mentally as Luke lie there late in the duel utterly a beaten man. And I didn't even need to mention the revelation to make my point either, but of course that is icing on the cake.
Trust me guys, watch the ESB Cloud City duel again between Vader and Luke, it is the only duel that has EVERYTHING!