SunRazer
Back From The Dead
Kun's the most powerful Dark Lord up to and of his time, which includes the Sith Lords I mentioned above:
"Once the most powerful and dangerous of the Dark Lords of the Sith, he [Exar Kun] was responsible for the deaths of millions four millennia before the rise of the Empire."-- The Complete Star Wars Encyclopedia
Ragnos was the greatest and most powerful of the Dark Lords, although, of course, you can dispute the meaning of "power" or "greatness" in these quotes:
"Marka Ragnos ruled the galaxy with an iron fist. He was the Dark Lord of the Sith — the most powerful of the most powerful. But now he is dead."-- Tales of the Jedi: The Golden Age of the Sith 2: Funeral for a Dark Lord (Publisher's Summary)
"Meanwhile, on the far side of the galaxy, the Sith Empire has grown powerful through centuries of dark Force wielding and magic and the hundred-year rule of the greatest Dark Lord of the Sith, MARKA RAGNOS."-- Chronicles of the Old Republic
Ajunta Pall became the reigning Dark Lord and the rest of the Exiles became his Shadow Hands, which puts him above them tbh:
"Ajunta Pall beheaded their ruler, Hakagram Graush, and claimed the throne as the blood heir to the Ancestral King Adas - and we became his Shadow Hands."-- Book of Sith: Secrets from the Dark Side
'We" refers to the rest of the Exiles - Karness Muur, Sorzus Syn, Remulus Dreypa, XoXaan, etc. And we know that Shadow Hands are second to Dark Lords:
"The title Shadow Hand originated on Korriban, indicating the second-in-command of a Sith Master."--Knights of the Old Republic Campaign Guide
"A Shadow Hand's expertise in cruelty, trickery, and the dark side are second only to the Master - for now."-- Knights of the Old Republic Campaign Guide
Again, you can contest some of these - the meaning of "power" and "great" in Ragnos' quotes, and whether or not Muur surpassed Pall at some point since we don't know the nature of their demises. But that's my evidence.