Marka Ragnos versus DN Luke
Who wins?
I say Ragnos.
He was feared by Ludo Kressh and Naga Sadow, two very powerful Sith Lords of the past. Sadow was known for conjuring massive illusions and also his greatest feat was triggering the explosion of a star. We see Exar Kun in a VERY small time frame generate blasts of energy that tears through temple walls and blasts through ancient Sith worms (gargantuan armour plated beasts of war) with Sadow's amulets. Sadow, far more proficient in Sith alchemy than Kun, desperately feared Ragnos.
In Jedi Academy, Luke mentioned that it would take the entire Jedi order to try and fight back at Ragnos if he were ever back in full fury. He had a fabled scepter that could drain the life from multiple worlds.
He was of the ancient Sith bloodline, a race fueled with the darkside.
He was described within the context of the ancient Sith (powerful magicians and force users) as having a terrifying grasp of the darkside.