Registered: May 2018
Location: Nazi Antarctic Territory
I say Jadus. He was said to be the most powerful Sith Vitiate's empire ever produced and was second in power only to the emperor. In the end it's tough to get a power scale for Jadus because there is limited information outside of quotes and dialogue about him power wise.
Although Darth Jadus was very powerful with a number of impressive abilities under his belt. The Outlander had his powers enhanced by Valkorion and he succeeded where others failed in dealing with the threat of Arcann and Vaylin respectively.
The Outlander is on another level in comparison now to the majority.