Lesnar via 1st round tko.
Honestly, giving Silva and shot against Lesnar is a bit absurd. In this hypothetical superfight, Silva would be giving up around 80 lbs to a world class wrestler.
Lesnar would take him down with in 15 secs of the fight starting. Stay chest on chest and pound him out from half guard. It would be ugly and Silva will look worst than Herring did. Hell, Ill even go out on a limb and say Lesnar has a chance of knocking Silva out standing.
Silva would be so preoccupied with a Lesnar takedown, Lesnar could fake and throw a huge right. When your fist is the size of Silva's head its going to be lights out.
Lesnar would win this fight 99/100 times. Silva would only win if he could somehow knock Lesnar out with one punch, which is doubtful.