It is Murray.
Most deserving for sure since it was not a surprise: Murray was going to win a slam, it was just a matter of time considering all his masters and slam finals. Del Potro has nothing in his resume to back his fluke slam, not before his win or after, nothing.
Most impressive? The Wimbledon final-Olympic gold-first slam at the USO sequence is a mighty impressive achievement. I don't think unexpected, or fluke, is a synonym of impressive.
Why are you factoring in everything else? This topic is only about the US Open of 2012. Did he deserve to win that tournament solely by his level of play? No. His level of play was crap. Was his win impressive? No.