For sheer drama and watching things become more and more desperate, Luke vs Vader in ROTJ. Their peverse game of cat and mouse around the stairs still impresses me, hence it gets my vote.
Obi-Wan vs Anakin in ROTS is great, it just gets more and more desperate and emotionally intense as it continues. And the end of that duel is absolute perfection, Obi-Wan's almost callous departure just emphasised the emotional wrack and ruin that this whole film has been spirraling down to. This would have gotten my vote if it wasn't for number 1.
The duels involving Palpatine were a marked let-down, Ian McDiarmid looked so awkward with the saber, and when he starting flipping around it seemed so fake. His and Yoda's force duel in the Senatorial Chamber made up for it though.
Obi-Wan and Qui-Gonn vs Maul is still number 1 in terms of physicality and visual flair. Watching Darth Maul wield that weapon with such consummate skill, fending off his two opponents at once just emphasised how much wasted potential there was in the character of Maul. And "Duel of the Fates" is a great piece of music.
I was very happy with one thing in the Dooku duel in ROTS, and that was seeing Dooku fight as I always envisaged him, finesse style, one hand on the saber, with swift deflecting cuts. And seeing Greivous in live action was worth it, although I have to ask, why did his wheeze suddenly vanish when he shucked the cloak? The final act of his duel with Obi-Wan was thoroughly entertaining, as they went fisticuffs for the last leg. And the whole theatre laughed at the attempted sweep.
"So uncivilised" - Obi-Wan Kenobi.