Darth Vader/Darth Sidious. Yeah it's hard for me to just pick one of them on one had Vader is the persona of badass on the other hand Palpatine is the greatest villain of fiction. Toss up for me.
Darth Plagueis. he is probably the second or third most powerful Sith lord to live. The control he had over Midi-chlorian was astonishing and him, together with Palpatine, created Anakin Skywalker. Not to mention Darth Plagueis is one of the best Star Wars EU novels I ever read.
Darth Zannah. Looking over it I liked her more then Bane, and she was a master at Sith sorcery
Count Dooku. Elegents, badass and Lightsaber master that's all I need to describe him.
Darth Bane. Creator of the Rule Of Two and straight up boss Sith lord.