Darth Thanaton. If Nadd was anything like Ommin he would need prep for his Sith Magic to take effect. Straight up fight Thanaton wins since he and the Sith of that empire were more focused on raw force combat.
__________________ "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."
Nadd was a very impressive lightsaber duelist, stalemating his master whilst he was a jedi knight, the former considered "A master duelist." He also does have in-combat sith sorcery, being what Zannah defeated Bane with.
__________________ ”You presume limits to my power. There are none.”
Ok, so here's my analysis of the battle, based off of what I know about these 2 characters:
We know that Freedon Nadd was able to stalemate a master lightsaber duelist, which is impressive in his own right, this is also coupled with his killing of many jedi with his bronze blade. Additionally, Nadd managed to prove himself to Naga Sadow by killing massassi, something even exar kun failed to do years later. Darth Thanaton, whilst a capable lightsaber duelist, has never been regarded as or defeated/stalemated a master lightsaber duelist. I give Nadd the edge here. In terms of force mastery, Nadd is very well-versed in the use of sith sorcery, and his teachings were passed down to Darth Zannah through his holocron, which Zannah later used to kill darth bane. He was also skilled in the use of dark healing. Darth Thanaton is also a very capable sith sorcerer, using a ritual-like attack mid battle to mentally subdue his adversary, although I don't know if this would seriously affect a dark lord of nadd's caliber. He also has very powerful force lightning, being able to destroy a large metallic statue, and has subdued (though not killed) a lenico gargantuan with lightning. He has also used it in a form of force flight, and as a FLS storm, though the latter takes time to charge, time in which nadd can dispatch thanaton with his sith magic-enhanced blaster. Ultimately, I give thanaton the edge here, due to his powerful force lightning, though the edge is not much. As to how the overall battle goes down, I declare Nadd the winner, due to having superior lightsaber ability, and his force ability is slightly behind, if not on-par, with thanaton's. This isn't even factoring in Nadd's sith magic-enhanced blaster.
__________________ ”You presume limits to my power. There are none.”
Hmm, I changed my mind. Thinking back to the Zannah vs. Bane fight, Bane, despite having more raw force power, was still given a difficult duel by his apprentice Zannah. The fear power is Waves of Darkness, isn't it?
__________________ "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."