Iron Man could potentially win if he made a suit out of Adamantium (that indestructable metal that Doc Ock's extra arms and Wolverine's skeleton/claws are made of) because the Mjolnir can't break it. Stark would be rich enough to make one.
I would imagine that would be very difficult to make and very time consuming. If those two were to go to battle right now, Thor would win because he's a god. Nothing much to say besides that.