its one of those questions that cant be answered...evolution will bring out better and better fighters as time goes on
instead, you might want to think about the fighters contributions to the sport of MMA in addition to his ability, the guys that changed the game. its obviously a much bigger list, or then its just another "who's the greatest (insert type of athlete here) of all time" question...
and in that case, over the years, i'd have to include folks like
royce, both shamrocks, severn, tank, ruas, couture, liddell, miletich, jeremy horn, maurice smith...the list goes on. they've all done much more than just win and/or dominate their fights