If you're going for raw impact and imposing size and weight, solid boron steel skeleton with tungsten carbide plating. It'll take one hell of a beating before going down. The weight will actually go up, however, with the Tungsten Carbide plating because it's one of the heaviest metals out there. Depending on just how tall this thing is. 

The joints themselves would require ridiculous amounts of strength, you'd need individual motors for every single joint. And I'm talking motors like in this thing: [img=http://i.imgur.com/VcLUwt6.jpg]

However, once you could get that weight into movement it'd be hard to stop. You'd need some kind of electronic control system that takes into account any deviation. Allowing the user to overcome this massive weight issue.