Lesnar appears to be unstoppable. I was actually surprised how well Randy wrestled with Lesnar in the clinch. There was a lot of strategic wrestling going on there between two masters. However, did anyone notice that when Randy tried to take Lesnar down that he could not reach around Brock's back. The dude is just too massive. Add his speed and he is a monster. The only heavyweights I see stopping him are Mir (who already did) and Gonzaga given their BJJ abilities.
However, people need to get their head's checked if they think he has a remote chance against Fedor.
Fedor would either knock him out (e.g. Tim Sylvia) or submit him in the first round.