Darth Revan:
Star Wars: Knights of the Old Republic
It's not often you get a bona-fide Sith Lord as a protagonist. Even then, the Sith part is optional. I'm going to go into spoilers, but chances are: you have either played Bioware's
Knights of the Old Republic, or you haven't and don't plan on it. So...
Darth Revan is the best Sith Lord. Hands down.
He (although it could be a she) has gone through a handful of masters, one of which is Kreia, the wise old hag and big bad of
Knights of the Old Republic II. He's learned much, he's been places most don't even know about, and he's probably learned more about the Force than anyone else. And that's before the events of the game occurred.
A short time before the game actually progresses, Darth Revan was kicking the collective backsides of the Republic with his scumbag apprentice, Darth Malak. Malak, being shortsighted, jealous, and jawless (presumably due to some disciplinary action by yours truly), decides on working towards his next promotion. The promotion itinerary for prospective Dark Lords of the Sith is as follows: Kill the old guy and take his title.
Since there's no way Malak can take Revan in a fair fight, he waits for the Jedi to step in. While Revan is busy fending off an elite strike team of Jedi on his flagship, Malak decides to shoot Revan's bridge with
his vessel's BFGs. Somehow, he fails even at that, and the Jedi escape alive with the incapacitated man of the hour.
Jedi: 1
Malak: 1 (to be revoked for foul play)
Revan: 0
Revan, while unconscious, was brought to the Jedi Council. Fearing they couldn't set him free, but also not wanting to kill an unarmed prisoner, they decided to mentally re-program him. Thus, the Player Character is born! Light or Dark, blade or force powers, you have hidden knowledge of basically everything, so you're the fastest learner in anything. Within a month, you're back up to considerable strength without any recollection of who you were.
An amnesiac Sith Lord.
From here, you choose whether to live your Jedi persona or become a Sith Lord (again). Even without conscious recollection of your previous life's experiences and wisdom, you're
still powerful enough to put Malak in his place.
Did I mention HK-47? Yeah, alongside being the best Sith Lord, you get to own the best droid in Star Wars history.