I'm still advocating
Darth Nox vs Barsen'thor (with all their resources at the end of the game*)
*not exactly, Barsen'thor before the ending ceremony and Darth Nox after the ending ceremony so they have all their resources at their disposal.
Edit: Another interesting combatant would be ESB Admiral Piett (including General Veers' ground troops).
Barsen'thor and Nox would have to both be the same AC based on assumption. Does Nox have the ghosts? Does the Barsen'thor have some kind of advantages like Force ghosts?
Assuming they're Sage and Sorcerer, we're seeing an interesting fight here