In terms of the actual physical fighting, I’d agree the ESB duel is better. Plus the whole atmosphere of the fight feels like something out of a horror movie, with Luke descending deeper and deeper into Hell throughout the fight until he can’t escape.

Personally, however, I prefer the RotJ duel because of what’s at stake in the moment. The scene isn’t really about the physical fight itself. It’s a spiritual battle between Light and Dark. The sword swinging is really just a backdrop to the struggle in both Luke and Anakin’s hearts against the absolute evil of the Emperor. And I always get chills when Luke shouts “Never!” and the chilling music starts. The choir that sings both during Luke’s outburst and when Anakin is about to save his son is really stunning.

In hindsight, I really wish the Anakin-Dooku fight in RotS had been able to rise to that same dramatic level. Anakin really has two pivotal moments in that movie where he starts his fall: killing Dooku, and stopping Mace from killing Palpatine. If I were a script doctor for the prequels, I probably would have tried to find a way to combine those two pivotal moments into one event, and tried to give that scene the same emotional gravity that Luke-Vader in RotJ had.