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.
Originally posted by NewGuy01
Much like Muur, Dreypa survived as a spirit for thousands of years via the use of an enchanted amulet,
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.
I'd be inclined to give it to Dreypa too.