I think most people here would agree that KOTOR didn't present 'the Dark Side' very convincingly. It's been one of the foremost complaints I've seen (and, for that matter, spoken up about myself.)
As such, which character, inside or outside Star Wars, do you think personifies 'the Ideal Sith'?
Remembering the debate I had when I first got here with Gromnir about Bioware's use of 'Johnny Quest villains' with no substance, it suddenly struck me that the KOTOR II team includes some of the writers of one of the most perfect examples of what the Sith should be presented as.
Thus, my choice: Planescape: Torment's Practical Incarnation.
*SLIGHT SPOILERS AHEAD!*
Absolutely perfect! Everything and everyone is a tool to him. He doesn't think twice about manipulating a young woman who loves him for his own benefit, sacrificing her for his own gain without batting an eye. But by the same token, he doesn't waste energy on things that don't benefit him; he doesn't randomly massacre civilians to no good purpose, though he certainly wouldn't hesitate for a second if doing so was to his advantage.
'Mercy' isn't neccesarily a weakness to him, because when he shows mercy to Morte on the Pillar of Skulls or Dak'kon in Limbo, he gains an advantage far beyond if he had simply left them to suffer. He essentially treats all of life as a giant game of chess, in which he sacrifices or spares pawns based purely on their use to him.
By contrast, if he had been a KOTOR Sith, he would have randomly let out evil belly laughs, randomly tossed in the adjectives 'pitiful', 'pathetic', 'worthless' to describe everything from his underlings to his cup of coffee in the morning, and thrown away advantages because he didn't want to show 'mercy.'