I never cared for the idea of more Sith post-ROTJ but I’m going with Darth Krayt. His One Sith was an interesting deviation from the rule of two. It took the combined forces of Roan Fel’s imperial knights, the remnants of the Jedi Order, Cade’s crew, and the Galactic Federation of Free Alliances to bring down Krayt’s Order. Darth Caedus was a poor attempt to rinse and repeat PT circumstances but with a character who was never meant to be a psychopath so I don’t think of him as worthy successor to anything.

^ What about Lumiya?

I say Caedus because on a cinematic level it would have been a beautiful thing: the son of Leia and Han who becomes the successor of Palpatine. But I must say that Darth Krayt is also very cool

^^Never cared for her character that much either since she was a part of the rinse and repeat of PT circumstances as well. The fact that Lumiya decided against corrupting Jaina because she was too emotional really puzzled me since that was exactly how Anakin was corrupted. Jaina had more than one moment where she displayed dark side tendencies but sure let’s go for the Solo who had no dark side tendencies to speak of prior to the legacy era lol.

Lumiya, then Caedus. Lumiya was trained by Vader directly making her the only candidate to continue the Rule of Two line since Vader and Sidious were the last Rule of Two Sith Lords. Darth Caedus next because she convinced him using Sith teachings and Caedus even made a sacrifice to become a Sith before he chose his Sith name.

However technically Lumiya isn’t a real Rule of Two and is more like a Dark Jedi with Sith Lord levels of Power and skill. Because she never went through the rituals of being hit with Sith lightning and making a sacrifice to become a Sith.

Darth Krayt I remember actually offended all the Sith Spirits he spoke to; being Darth Bane, Andeddu, Nihilus. With Darth Bane directing saying Krayt and his order were not “Real” Sith. And all other spirits refusing to help him. So Krayt’s definitely not the successor. Although I think him and his order were actually quite cool.

Why is Krayt here? He wasn't a successor considering the proprietor of the Rule of Two Sith, Darth Bane, flat out stated he wasn't. Calling him a pretender, Lord Andeddu called him a Heretic.

Krayt created a NEW Sith Order entirely, not sure that would fall under being a Successor to the Rule of Two considering he didn't continue the Bane Line and instead made his own Order.