I would have to say Anakin versus Obi-Wan. Probably because I couldn't comprehend the emotional intensity when I first saw the Luke/Vader duel for the first time. (I was like 5 or 6).
Luke vs Vader was Vader trying to seduce Luke to the Darkside--with Luke taking in a somewhat exhilirated and flat tone, hardly emotional. Then it switched to Luke getting all pissed and hacking at Vader, also not emotional.
Obi and Vader had very little talking until you get a hopeless Obi saying he lost and failed his "brother" and said "brother saying" he hates him. Plenty of emotional voices and faces.
Obi and Vader are much more heart-rending.
__________________ Recently Produced and Distributed Young but High-Ranking Political Figure of Royal Ancestry within the Modern American Town Affectionately Referred To as Bel-Air.
The emotional icing on the cake for me (in the ROTS battle) was when Anakin was sitting there, burning, saying: "I hate you!!" And Obi-Wan replied: "Your were my brother, Anakin. I loved you." That last little bit right there added to the emotional intensity.
I can hardly remember Luke fighting Vader in ROTJ, I only remember him being electrocuted.
Although Obi/Anakin didn't impress me that much I think it's still a better fight.
__________________ Recently Produced and Distributed Young but High-Ranking Political Figure of Royal Ancestry within the Modern American Town Affectionately Referred To as Bel-Air.