In a boxing ring, Tyson; especially in his prime (duh). Knock Poundstone through the ring floor.
In a street altercation, my money is on Poundstone. Iron Mike could get, again, that lucky and decisive punch in, but Derek would likely smash Tyson into jelly jam.
His size and strength would be too overwhelming. But he would have to be very guarded when he moves in to protect against "killing" strikes from one who is a master. If he took one in one of the sweet spots, it could be over quickly.