I am not too expert of any of these characters, but nonetheless I voted Count Strahd, Vecna and Szass Tam.
I have read the basic Ravenloft handbook, and Count Strahd gave me the impression of being evil in a very sophisticated way, not just the dumb BBEG who "wants to rule the world", but instead a much more realistic and dramatic evil, and therefore more impressive. I have never played Ravenloft, and still that setting gives me a feeling of being somehow restrictive, but the evil guy is really interesting.
Then the other two, which seem to me very similar (I suppose they could be the same person, with Vecna advanced to godhood), are also very scary. They are IMHO two of the best lich-types I've seen.
Ok, maybe the poll was about who's the meanest, but I actually picked who I think is more interesting...