It would depend.
One who views Naga Sadow as the greatest we've seen would put one down for Marka Ragnos.
Apparently, Exar Kun was the strongest Ancient Sith (Someone said KJA said so). He can be argued.
Then there's Luke.
I'm unresolved on this. It's really depending on your views.
(heh, anyone notice that a lot of the "generally accepted" most powerful characters come from TotJ? I'm guessing because comics are generally more free and not limited by "cheesyness"😉