Correction: the Fatman server was named after a ship in the Deceived novel.
My vote goes for Darth Malgus. In the Deceived novel, that guy is just a beast. The best part is in the end. I won't give spoilers, but he basically walks through this guy's force lightning like it's nothing.
He's also a true Sith in my opinion. He is all about conflict, whereas other Sith wanted the Treaty of Coruscant. He really is a true Sith. He makes decisions based off of the Sith Code. Also I think Malgus acheives oneness with the Darkside at one point. That's how epic he is.
His fighting style is all about strength, so I imagine he uses the Djem So dueling style. Just throwing that out there...
I confess that I haven't played through the Sith Warrior story all the way - so I'm a little ill-informed when it comes to Darth Baras. I'm sure Baras would make it a close fight however. I've heard he can get EXTREMELY angry, and anger is power for a Sith.
He is a true sith, however not because of that. The otehr sith lords didnt wanted the treaty, the emperor wanted it.