Slight spoiler.....
Well up until AOTC I would have said the final duel in TPM, but after having seen AOTC it has to be the hangar encounter (Dooku, Obi-Wan, Anakin, and of course Yoda), with the highlight being Yoda!
I thought we suddenly saw Dooku go evil, he didnt seem that bad for the first half hour, but when he began taunting Obi-Wan:
"Master Kenobi you disappoint me. Yoda spoke so highly of you."
"Come on, surely you can do better."
Christopher Lee really seemed to bring a ferocious evil to the screen and that made the duel that bit extra fun to watch. Then of course Anakin V Dooku and the flashing lights and that, making it hard to see, that made it more intense.
Then of course beforehand when we saw Yoda and we just heard him say:
"Good, very good commander. Bring me a ship."
I was hoping he was going to join in. Then we had the best encounter ever seen in a film, Yoda Vs Dooku, that was just class.
So definately the Hanger Encounter in Episode II, it made Yoda my favourite character.
TPM duel. No contest; it was protracted and virtually thr whole of the end of the film was built around it.
The AOTC duels were nice but I doubt they were designed to be the equal of TPM; certainly less time will have been spent on them. The centrepiece was the arena fight, of course.
I would imagine that ep. iii will top TPM, though.