My vote is with Dreypa currently. He's absolutely mad, but he was a rival to Karness Muur, and his Force storms ravaged an entire village of the Lost Tribe.
Much like Muur, Dreypa survived as a spirit for thousands of years via the use of an enchanted amulet, and had control over an army of leviathan monsters.
Overall he seems to be a head and shoulders above Thanaton, being one of the original Sith Lords and a contemporary of Xoxxan and Sorzus Siin, who's teachings were valuable to greats like Palpatine and Krayt.
And a stasis coffin that made sure his body was alive.
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Thanaton's lightning is great, but Muur also possesses really powerful lightning so Remulus certainly must have some defense against it, and Dreypa seemed fairly physical.