I don't read much pre-original series stuff, because I think a lot of it is crap. So I'm just going to go with Exar Kun. Maybe he wasn't technically a Darth, but Dark Lord of the Sith is close enough. He killed an entire race of people just to drain their power and bind his spirit to the temples of Yavin 4. That's pretty hardcore. :p
If the new Legacy of the Force series continues on the path it's on, I think (SPOILER)Jacen Solo may just become the most powerful Sith Lord/Darth ever. ;) They probably won't let him, though. Too many fans would be upset.