The head is pretty much made to take a beating and protect the brain as much as possible, ~~so I don't see any single direction it would be better at~~ but of course the forehead is better than the other parts. Maybe headbutting played a role in evolution?

Shoulders, from what I see, as long as you keep the muscles tight (and not get tired doing so), should give similar results in all directions, though having the impact taken by the limbs first, would significantly reduce direct damage to the joint.