Do you think Exar Kun could have taken on Marka Ragnos while he was alive? What about Kun against Revan?
I dont know really, when you bring all these powerful sith lord together and try to compare who really was the most powerful, it becomes very difficult because they were all OP in their own ways.
Exar Kun: Incredibly powerful duelist and a jedi turned Sith Lord, claimed to be one of the best light saber duelers in star wars universe. Very high affinity to dark side magic and the force. Took thousands of jedi to stop his spirit form after he found a way to become non-corporeal through essence transfer. Trained by the famous Jedi Master Vodo Siosk Baas and later by the force spectre of Sith Lord Freedon Nadd.
Freedon Nadd: Another jedi turned Sith Lord. Also very powerful in dark sorcery and trained by Sith Lord Naga Sadow. Created a dark side empire on planet Onderon and then became a force spectre upon death. Seeping the entire planet in dark side energy.
Naga Sadow: I dont need to say much about this guy besides the fact his force illusions are one of the most famous in star wars lore, his dark sorcery was legendary. Created entire fleets of ships out of thin air through force illusions in his conflict with The Republic in The Hyperspace War. This dude was also fighting another Rival Sith Lord (Ludo Kressh) during his entire war campaign aswell.
Marka Ragnos: Known for being one one of the strongest dark lords of the Sith to ever exist across history. He created one of the first and strongest sith empires and later became a force spectre upon his death. Passing the torch onto powerful dark lords like Naga Sadow and Ludo Kressh, telling them that only the strongest, may succeed him. Absolute Legend.
Darth Revan: The big meme of star wars for being incredibly powerful and gifted in the force beyond all belief. Whether using the light side or dark side, the natural talent and affinity for the force made this guy damn near unbeatable.
Darth Nihilus: Good luck trying to kill this guy, another non-corporeal being that actually wrecked havoc in his energy form. He didnt care about the sith or jedi or anything else besides just eating. He kept on Lardmaxxing on the force and became ridiculously OP, sucking the force out of every living thing he encountered, but the problem was that he only got more and more hungrier.
Darth Sion: Aka - Mr Immortal. The 'Man too angry to die' meme of star wars, the Doom Slayer of the dark side. Literally no point trying to fight this guy because he will respawn and slay you down. One of the Dark Lord partners of Darth Nihilus.
Darth Vitiate / Valkorian: Another competitor for the title of the strongest dark lord of the sith to ever exist. The most famous practioner of the Dark side essence transfer which basically allowed him to live forever. I basically dont even know how long he ruled but i know it was very very very long. Shat on the jedi, shat on rival sith dark lords, godtier manipulationmaxxer. Had multiple assasination attempts on his life but this dude just could not be killed. Absurdely OP in the force.
Theres probably many more sith i could discuss like Yung homie Palpatine, Lord Vader, Darth Kryat, Dooku, the list goes on. I would fall asleep. Its very hard to determine who is the most powerful just because of how the whole universe is laid out.