As far as greatest ever, to me, you can only be fairly compared with your era. Some people will short-circuit over this because the worship of Fedor Emelianenko is like a religion and people hate to hear that there is no God, but this Cain Velasquez beats any Fedor Emelianenko eight times out of ten. Emelianenko's takedown defense was nowhere near that of Dos Santos, nor was his stand-up as good or as versatile. His footwork wasn't close to as good, and Dos Santos as a physically bigger and quicker athlete.
Emelianenko did hit hard and could win. He had a submission game but he's not submitting Velasquez, and he's not outworking him to win rounds, nor could he match his conditioning. Realistically, he lost to Dan Henderson and Bigfoot Silva and there is no way, even at his best, he could have gotten past Junior Dos Santos, other than the same puncher's chance.
Emelianenko did fatten his record on a number of circus opponents, but that's what Pride was, essentially Japanese pro wrestling without the fights being fixed. You can't knock him for his era or his surroundings. All you can do is beat what is in front of you. Emelianenko did so from 2000 to 2009. Not many of his opponents could have survived in today's UFC, but he did beat Antonio Rodrigo Nogueira twice, who was clearly the No. 2 heavyweight of his era, and his win over Mirko Cro Cop was the great heavyweight fight of its era..
So as an all-around fighter and who would win head-to-head, I'd go with Velasquez.