Hmmm....kind of a tricky thing to think about. The Tyrannus arguement is tempting, but remember a few things. It was mostly
because of Dooku's
death that Sidious went hell-in-a-handbasket for Anakin Skywalker. Of course, the other side of that coin is that Dooku was a pawn from the start as Palpatine always had his eye on Anakin from the beginning.
Also, I'm not sure if Dooku would have really become a full-fledged Lord of the Sith in the traditional meaning. Remember, he was manipulated as much as Anakin was. He truly believed that the Senate was corrupt and the Jedi were in the dark and powerless over the whole thing. He saw the Jedi as important and noble, still holding his respect for the Order, but felt they had become mired in thier own dogma. So, he would have a been a bit "on the fence" to be a real Sith-Lord-In-Charge.
That being said, he's still the best choice out of the ones provided. Maul was little more than Palpatine's private assassin, Ventress was very confused and maybe even
too angry and vengeance-obsessed to be an effective Sith Lord. And Grevious was no Jedi to begin with, just a pseudo-droid who was good with lightsabers.
All of that being said, I think the helm of the Sith could be held by a little-known fallen Jedi with a complete understanding of both sides of the Force as a whole and an allegiance to no one. A powerful individual known only by a select few on an obscure fetish-related message board...
...Darth Furyan.

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