Tyson. Why? Every fight begins standing and the only way for Lesnar to win would be to take Tyson down. Mike is a much more powerful, faster and accurate striker than anyone Lesnar has ever faced in mma. In addition to that, he's as strong as a bull and very aggressive. Also, he's short, meaning his centre of gravity is lower, so he is harder to take down than tall people. If Lesnar decides to stand and trade even for a few seconds, he gets KTFO cold, no question. If he attempts a takedown straight away, I think he would eat a massive uppercut/hook closing the distance, knocking him out. If he did manage to take Tyson down he would win though. Lesnar was always in deep **** if his opponent had any decent striking ability e.g. Carwin, Overeem, Velasquez... I think Mike Tyson wins this 7 or 8 times out of 10.

Boxing- Tyson by KO RD 1 within 30 seconds

Streetfight- Tyson by KO 1 Punch.

MMA- Brock was a 2x NCAA Wrestling Champion, and would have a chance, but Tyson would probably still knock him out.

Brock Lesnar would take Tyson down and beat him senseless in about a minute or else he would pick him up and throw him out of the ring.

Not even close Tyson in the first round

If it is boxing, I have no doubt about Tyson beating Lesnar as long as authorities make sure that Lesnar is not on anything illegal in boxing.

Lesnar can't even fight. Even if he was a UFC fighter. Tyso KO in the first 1st 10 seconds

Tyson in his prime would destroy Lesnar.

Lesnar has never been in a ring with anyone even approaching Tyson's ferocity.

Tyson - I don't rate scripted wrestlers.

tyson will iron brock flat!

That would be an interesting fight to watch , I think Brock would knock mike out though. But we'll never know till they face each other (that's if it's ever gonna happen)

Who would win? Mike, a former heavyweight champion a true vicious fighter and is not a afraid to let himslef go. Brock, a former hevayweight champion and in WWE( yes I know its scripted) is a beast and just goes for it. Personally I think Tyson would win