Alone, all the Sith Lords in the game are flawed, but together, they embody the Hunger, Pain, and Treachery of the Sith. Of the three, I think Sion is the best. He looks freaky and is, as it has been said before, the only Sith truly loyal to Sith ideals: pure servitude of the Dark Side and a bloodlust for Jedi.
I think it would have been cool if you fought some sort of Sith spirit at the end of the game, formed from the souls of the Triumvirate and having all their powers. It would have been the perfect Sith, with all three 'virtues'.