well my vote would go for either Malcolm Mcdowell [from clockwork orange- met him once but thats besides the point lol] as he's do one helluva job at creeping everyone out. Or after watching the film Phone Booth - Keifer Sutherland. [if you've seen the film you'll know what I mean by his voice, to my mind it proves he could carry it off].
But Mcdowell is still my first choice. Although Grevious isn't a sith [unless GL has serisously f**ked up his rule of 2 thing!] He should still have an over bareing presence in the film like the other sith we have see so far. Quietly spoken but with a voice that carries a sense of power.
Anyways thats my views for what they are worth.
Wayne...