Garry Kasparov. He dominated his era, and his opponents weren't push-overs...
Kramnik has an equally impressive record against Fritz(computers) as Kasparov, if not more so. It will be interesting to see how he does in the upcoming championship. Will he continue from where he left off after his impressive win over Kasparov?
You know, it is difficult to compare the games of players from different eras. Dominance in one generation does not mean dominance in future generations. I would say that there are more higher calibur players as a whole right now than there has been at any other time in history. That would tend to make one believe that the level of competition would be higher and more difficult to dominate. I think it would be easier (and possibly more accurate) to name the top 10-20 players in history. It would be impossible to know who the best would have been because it is impossible to have them all play each other in identical settings. That is my take, but wtf do I know?