The answer for me is easy and its Roger Federer. Although he has only a 1-1 record against Nadal and a 1-0 record against Murray (he hasn't played Djokovic this year), hes won 4 titles and he has 2 AMS titles.
He's been pretty much unbeatable aside from two matches this year against Rafa in Melbourne where he inexplicably broke down after the firework delay and in DC where he lost to Isner. The Roddick match obviously counts but it was a jaded Federer
I think its between Fed and Nole as with the level Nadal shown being sub par.
Inexplicable? No it's not. It's so easy to see Federer just can't elevate his level as much as Rafole. They are unstoppable at slams, and I'm sorry, but Federer can rule all he want, when the money is on the line, he gets dealt with by the actual best two players in the world