Tailback U
No substitute 4 strength.
Speed + Power + Technical ability = dominance. Fedor has it all. Brock has power and is a great wrestler. Right now, the UFC really has no competition for Lesnar. Cain Velasquez is a great wrestler, but, his striking and stand-up need more work. Nogs may be the default #1 contender depending on what happens with Couture at UFC 102. All in all, there just isn't much out there for Lesnar now.
A matchup of Fedor vs Lesnar would create huge PPV numbers! However, Fedor would make quick work of Lesnar. Fedor would basically take that horse shoe that Lesnar pulled out of Mir's ass and shove it down Lesnar's throat and pull it out his asshole...and then fuck Sable for good measure.
Fedor > Lesnar
I'd love to see it though.
Here's the thing, Brock is way too big and powerful for pretty much every HW out there. His wrestling is great and he can shoot and takedown anyone. So for somebody to beat him, they are going to have to be very good on their back and very calm because it is not easy for anyone to hold guard on a 290 pound albino gorilla (stole that from someone here and love it).
I think Fedor would tap Brock because he is the greatest ever but it wouldn't surprise me if Brock was able to just overwhelm Fedor with hammerfists because he is just so fucking big and strong.