As much as i hate to admit it, Djokovic is by and far the clutch player by miles as he has shown since USO 2010. The number of matches he has won after being match point down or within a point or two of losing are too many to count.
Nadal has not gotten himself into such situations often.
Federer has some 9 losses including 4 at majors losing matches after holding match points.
So , on this count , it is hands down Djokovic.