I agree with anyone that says that ESB's duel was great. It definitely had the most feeling directly associated with saber-fighting in the entire series. Otherwise, I thought RotJ's lightsaber duel was a lot more intense and accented the other tensions in play and thus that's my favorite duel from the OT. Though, I also love Ben and Vader's duel in the original Star Wars.
TPM's duel of fates was the most intense saber-fight in the series for me and thus entertaining, mostly because Maul seemed so incrediby intimidating during that whole part. . . then he kind of died like a wimp (oh well). Sure, it wasn't as good compared to the OT duels for me, but still very cool I thought. The final battle between Obiwan and Anakin was also able to hold my attention at least. I believe the rest of the PT duels were pretty much crap however.