Round 1: SoD Maul - around 8-9/10? Within Canon it's depicted pretty clearly that Vader is Maul's superior. Vader's strength provided greater issues for Ahsoka obviously, his Force feats are superior, he demonstrated superiority in every regard so far.
Round 2: Count Dooku - here I can't tell. Dooku is simply more estabilished, his feats within lightsaber dueling are just better, and Force power wouldn't account for much. By the time of Rebels Vader is pretty hard to gauge in comparison to his Skywalker self (and, by extension, to Dooku).
Rount 3: Darth Malgus - 0/10, obviously. Malgus can fight groups of people that alone are within Vader's paygrade, and Vader has nothing to imply he would survive a Force Crush from Malgus or Lightning barrage.
Round 4: Darth Tenebrous - 10/10. Besides Rule of Two powerscaling Tenebrous has nothing going for him. Tenebrous was the lightsaber enthusiast; Plagueis was not. Tenebrous had every knowledge about Plagueis's dueling, being the source of that knowledge; Tenebrous could hide many things from Plagueis. Tenebrous trained Venamis to kill Plagueis. Venamis was created as a work-around precisely against Plagueis's fighting style, Venamis was basically the best response to Plagueis Tenebrous could create. Venamis was never seen outmatched physically by Plagueis... yet Venamis failed. Tenebrous's most masterful Force application, maxi-chlorian manipulation, turned out to be utter failure.
Round 5: Darth Krayt - 3-4/10. Krayt is slightly better in combat skill, Force power, mastery, Force usage within combat, has greater array of possibilities.
Round 6: Exar Kun - 4-6/10. Can't tell.
Round 7: Darth Caedus - 0/10. Caedus is better in everything; sometimes just a notch, sometimes a whole level. While physically there are marginal differences, Caedus is more powerful, more masterful in every Force application, more masterful and capable in usage of direct Force power within combat situations, has access to many Force techniques beyond Vader, is definitely a superior combatant in every field. I can't see anything allowing Vader to win a single round.
Round 8: Darth Plagueis - 0/10. Yeah, similar as with Caedus. Greater disparity in the Force, lesser one in lightsaber dueling, that's it.