I think Brock Lesnar could put up a good fight for a lot of the lower ranked HW's in the UFC... guys like Kevin Jordan, Branden Lee Hinkle, Brad Imes, Gabriel Gonzaga, Fabiano Scherner. That doesn't say anything good about Lesnar's skills though.
If I was the bookie, I'd set up Lesnar vs. some middle ranked HW... Assuerio Silva, Buentello, or Eilers. I think Pe De Pano might be a good match up too, but I'd bet on Cruz, not Lesnar.
Guys like Arlovski, Mir, Monson and Sylvia would annihilate Lesnar, who just doesn't have the striking technique or speed. Lesnar outwrestle Mir or Monson? Yeah right.
I can picture it in my head right now. Lesnar goes in on one of the top-ranked HW's, and tries to pick him up for the bodyslam. What happens? Lesnar gets choked out. Guillotine. End of Lesnar's MMA career. I think Goldberg stands a better chance, quite honest.