Game: Mike Tyson's Punch-Out!!
Mike Tyson was the final opponent in the original Mike Tyson's Punch-Out!! It was protagonist Little Mac's dream to fight Mike for the title. Eventually Tyson's licensing deal expired and Nintendo (in one of their least creative moves) simply renamed him "Mr. Dream" and like another famous Michael with legal problems turned him from black to white.
Tyson was not only, by far, the toughest opponent in the game (landing any of his punches in the first 90 seconds would knock you the hell out) but also the most frustrating to lose to. Being defeated by any other boxer was bad enough but you always had a chance for a rematch. However if it was Tyson that delivered the knockout blow it resulted in an automatic game over.