I'm going with Sion.
He has beaten Lonna Vash, who was Jedi High Council Member. He's also slain numerous Jedi, to the point where he became a feared lightsaber assassin:
"Darth Sion has vanquished many Jedi foes."
―Source: Knights of the Old Republic Campaign Guide
"A feared Lightsaber Assassin, Sion's chaotic wrath is directed towards Sith and Jedi alike."
―Source: Insider #88: Heritage of the Sith
He's also one of the strongest Sith of his time and one of Surik's most powerful enemies:
"Only by fighting your way through the Sith's strongest guardians can you earn the right to confront your nemesis."
―Source: Knights of the Old Republic II: The Sith Lords: Prima Official Game Guide
"At last, you have defeated your most powerful enemies and can finally rest and take comfort in the fact that the Force itself is safe from Kreia and the Sith (for now)."
―Source: Knights of the Old Republic II: The Sith Lords: Prima Official Game Guide