Speaking of MMA globally it has to be Fedor. Guy was undefeated for 10 years in his prime time, as a freakin heavyweight. And literally defeated all the best fighters at the time. Coleman, Minotaur, Cro Cop, Arlovski, Rendleman, Mark Hunt and many more.
As of UFC I would say GSP. If Jones wasn't a cheater I would possible compare it to him, but only "if", who knows if he would still be that successful. UFC always had double standards. Speaking of GSP, he is just amazing both as a person and a fighter, best MMA representer of all time.