Darth Tenebrous at least was the same skill level as Windu when it came to dueling. Mace has strength incorporated into his dueling while Tenebrous would have been stronger due to his cybernetic enhancements. He also was calm and collected in lightsaber dueling just as Dooku is. The second he saw a weakness, he cut you. He was also said to have been fast in dueling regarding his prosthetics due to force speed. He used this often so his would be more powerful than Windu's in my opinion.
__________________ "Vader's pulse and breathing were machine-regulated, so they could not quicken; but something in his chest became more electric around his meetings with the Emperor; he could not say how. A feeling of fullness, of power, of dark and demon mastery -- of secret lusts, unrestrained passion, wild submission -- all these things were in Vader's heart as he neared his Emperor. These things and more."
Name some feats, not any accolades, we don't call someone a "great" duelist until he's been shown doing some "great" dueling.
As far as I'm concerned, Windu murders him in a duel, he's one of the greatest duellists in the entire mythos, and there are very few, if ANY people, that can match him a pure saber duel.
1. Plagueis was very powerful in lightsaber dueling saying that he can run through the trees in the battle with Venamis as though he was lightning. Tenebrous taught him that.
2. Venamis was given some of Tenebrous’s dueling skills and he could overpower plagueis in dueling.
3. It was said that he was the best lightsaber duelist in the Banite timeline before Sidious. This is known because he was known to train only in lightsaber dueling and not Sith sorcery.