I would allow myself to say: Darth Caedus, for being I think the only one who didn't rise to power through the law, but rather by changing the law. (Lawyer droids...YUCK)
However, Darth Bane would be the Sith Lord I'd want as a master, for he would not kill you until you were ready to kill him, and he had no sense of betrayal as Sidious did. Darth Revan was cool and all, but Bane maximized his teachings through the holocron to become one of the more powerful Sithies.
Plus: Bane marked Sith disappearance. Sith, Sith, Sith, Sith, no more Sith. Jedi say wha?
Sith come back after a millennia, and FAIL utterly FAIL, I blame Sidious for being the Sith who broke the chain and killed his master too early.