I'd say the greatest active writer of sf/f would be Gene Wolfe or Ursula Le Guin. For "greatest living" I would divide the crown three ways and include Vance--maybe four ways, including Fred Pohl.

I like the people on your list a lot, especially Mieville and Bujold, but I think some of them are too early in their careers to claim top honors.

That's obviously not true of Pratchett and King. And I'm not knocking them: their place in the pantheon of genre fiction is secure. But their best work lies in a fairly narrow range (horror for King, humor for Pratchett). It seems to me that the greatest writer in the field should be less specialized.