Vader didn't actually loose his battle with Luke, in fact if you really read into it he wins his battle, it's what Vader had been trying to do all along for both Empire Strikes Back, but he was caught off guard when he found Luke was fighting with more force than he imagined. The Return of Jedi fight could easily have been won by Vader, how easy it would have been for Vader to just kill Luke, he left himself extremly vulnerable every attack he made (over handed attacks leave the legs open for an attack), though that was not what he wanted, what he wanted was to turn his son so he could stand together with him. That has always been Vader's downful his loyalty to those he loves.
Vader could liquify Revan with a mere gesture, maybe he'd torture him a bit, or possibly even battle with him just to quench his need for hand-to-hand combat. He is the greatest Sith Lord ever to walk, he is more powerful than the Emperor even, the only reason he hasn't killed the Emperor before was because he knew that the slightest elctric shock to his breathing devices would mean ultimate death.
Exar Kun would be more of a challenge to Vader, though he'd still be able to conquer him, especially if Vader took him on in combat, for he has no equal in combat before or after.
As you can tell I adore Vader. I only wish he lived for longer, so there could be a longer story to him than just the films