When I joined this site, Rait and MarekP were the best, so they'll remain that way in my mind. As discussed in that recent GR thread, it takes playing a person more than once to get a good sense of if they are all that good or not, people can have a random bad showing, or a random good showing! So, my list is mainly of people I've played at least 2 or 3 times, typically more, and I'll put it in alphabetical order:
Akroma, Babak, BigZombieDude, Bugger, Crazyter, davebishop, djbent, EdiBirsan, Ivo_ivanov, MarekP, martinck1, orathaic, Rait, rdrivera2005, slyster, TheGhostmaker, TheWizard, V+, zultar.
There are others that I suspect should be on the list, and will be soon, but I need another game or two before making it official (especially since I've been wrong before)! ;-)