C3PO: Siths, Korriban, and the Dark Side oh my!
Yes, we all know THAT Threepio.
(Just so you know, I am NOT a Sith person)
Well, here I go into the realm of the darkside(ACK!)
So, to answer the question, I'll list my three favorite.
I know, I know not very original, but Darth Vader is my favorite Sith. Its most likely because of the fact that he was Anakin before, but I liked him even before I saw the Prequel trilogy. Really, if you ask me, I don't think he was ever a true Sith. Yes, he WAS a Sith, but from reading some books such as Dark Lord: The Rise of Darth Vader you can see that he doesn't have the sickening obsession with the darkside; he wants to Master but doesn't seem as fanatical as Sidious or Bane. I also love the fact that, in the end, he was able to break away from the Sith, something rarely done, and returned to the Light, defeating Sidious and saving his son. And in the end, the galaxy is set right, he is reunited with Obi-Wan, and is the Jedi, Anakin Skywalker, once more :).
My second favorite Sith is Darth Zannah, apprentice to Darth Bane and eventually, Great Lord of the Sith. Darth Zannah starts out as a little girl left alone on war-torn Ruusan who is found by Darth Bane- the only Sith left alive. Sensing her remarkable connection to the Force and her anger and pain, he says he will train her, but first she has to prove herself.
Zannah is an interesting character; she's an evil Sith but sometimes you can sense humanity in her(well, at least, in the second book anyway. It's gone by the time the third one rolls around). I also like her because she's a girl bad-guy who isn't inappropriately weird *cough cough Darth Githany*. She is also a very powerful Sith and can fight really well(she also is a user of Soresu and fights with a double-bladed saber. And finally she defeats Darth Bane in their duel(who is the Sith'ari) when he tries to take over her mind. All in all, she is a really powerful Sith.
My third favorite is Darth Revan. I don't know much about him and what I do know about I learned from Wookiepedia, but from what I read, he is a really neat character. He was a Jedi-turned-Sith-turned-Jedi again. He also ended up falling in love with Jedi Knight Bastila Shan; which is really weird because they both did the back and forth thing and both helped each other out in coming back to the light. Though, it is really cool.
A book has been published on him and I plan to read it really soon! Can't wait!! :).
There you have it; my three favorite Sith Lords. Now, if I had to go with the most powerful or the most evil Sith, I'd go with Darth Sidious or Darth Bane.
Tune in next time for more Star Wars fun!
May the Force be with you!
Signing off :)