I agree with chilled monkey on this one. Ancient Jedi and Sith could do things modern Jedi and Sith could never dream of; knowledge of the Force was lost over time. Heck, Darth Sidious, one of the most powerful of the modern Sith, got his butt royally handed to him by the ancient Sith, enough so that his flesh decayed and his Hands desperately tried to save his life (even went to prepare a clone for him), and the ancient Sith weren't even actively trying to kill Sidious; they merely insulted him in DE.
Once Obi-Wan defeats Darth Maul by simply walking into the room and using the light side to kill him, then he might be getting close to Arca Jeth. King Ommin was a Sith sorcerer, and he DEFEATED Jeth. I doubt any but Yoda, Luke, or Mace would be a match for Ommin, and even then it would be a very, very difficult battle for them.
I mean, Ommin incapacitates foes from miles away by concentrating the dark side power he saturates throughout a planet's atmosphere. He received training from Freedon Nadd himself; to see just a fraction of his power check out his story in the TOTJ companion. And, if one wants to use the dark-side-corruption-means-greater-power argument, then Ommin certainly wins out over Palpatine there.