I think the character that best personifies the ideal Sith (even though I am not a Sith, I have extensive knowledge of almost everything) is Naga Sadow. He was a true member of the Sith species. He bound the Force to his will and was one of the first to use the Dark Side of the Force properly. He dabbled in alchemy and with that created not only monsters, but weapons. Naga Sadow may not have been the strongest or most powerful, but he is a real master of the Dark Side. A true Dark Lord of the Sith.

Actually i think that Irenicus from BGII would make a rather splendid sith lord.