IMO Mir has some of the best groundgame if not THE best in the heavyweight division, the trouble is I don't rate his stand-up and I think his conditioning is awful.
As for the Royce Gracie, he won the first UFC's as nobody had seen such a ground game but fighters have evolved and used a mixture of boxing, Jui Jitsu, judo, wrestling and various other techniques to improve. This was shown a couple of yeasr ago when Royce was beaten by Matt Hughes, at the first UFC's Royce was beating men who were bigger and stronger by technique, however fighters have moved on and Royce was dominated by a smal