It should seem obvious that the answer is Fedor. I understand the respect given to weight classes. But The Greatest of All Time should go to the dude who was the living breathing baddest man who ever walked into an MMA octagon or ring. That's Fedor Emelianenko. Sure, we see the present far more clearly than the past. We are myopic in our thinking sometimes. But from early 2000s to 2011, Fedor was almost a ghost story. A myth. A spook story you told your children. A special attraction. The boogieman who came out, with that blank, muddy stare...and mutilated human beings...with a facial expression that looked like he was doing a crossword puzzle. He was worse than a monster. A monster is big scary, snarls a lot and growls and is terrifying. Fedor was more like whatever it is monsters have nightmares about. Not since Mike Tyson has anyone had such an aura of intimidation. I'd say Jon Jones, who is ostensibly undefeated for 12 years, against great competition, has to be 2nd, followed by Anderson Silva who was a terminator for about 6 -7 years. Then GSP, who was smaller, would be 4th. A prime Jones vs a prime Fedor....I'm not sure who would win that one. But since that one is pretty much a stalemate, I give it to Fedor since he was a force of nature.