Finished reading "Revan" from the old republic, and i don't really think any sith or jedi in history can top Lord Vitiate. He enslaved both Revan and Malak, even if just for a short time, he broke them mentally, and turned them into his puppets. He completely consumed all life on his home planet of Nathema. He still beat Revan, and Meetra the Exile (admitted with little help from Scourge, but none the less, still beat them) Fed off the power of Revan. He has been closer than any Sith lord ever to achieving true immortality. He brought the Sith Empire back into existence after the Great Hyperspace War, he convinced Madalore the Ultimate to go to war with the republic. His voice, even as a child, carried the weight of the dark side. After he consumed Nathema, his voice seemed to carry the tortured souls of the millions of living things he had annihilated. In the end he was beaten, but you can't deny that he had accomplished more in a thousand years of rule, than any force user before him.
The very existence of flamethrowers proves that some time, somewhere, someone said to themselves, "You know, I want to set those people over there on fire, but I'm just not close enough to get the job done." -George Carlin