I'll say Wimbledon, especially after that loss in RG I thought he would lose steam for the year, but he proved me wrong. But I would say Madrid was a close second for me, I really did not expect him to take out Nadal on clay.
My first thought was Wimbledon but after thinking about it for a bit it's probably Madrid or Rome. For him to actually defeat Nadal on clay was more surprising than Djokovic winning Wimbledon after he had already been so dominant the first half of the year. And especially for Djokovic to defeat Nadal in back to back finals on clay in without dropping a set was quite shocking.
None... I knew he would reach the road to slam titles after the 5 set victory over Federer last year, that was the major turning point in his career. The only thing I did not know was that he'd have a ridiculous streak like this.
It is clearly between Wimbledon and Rome for sure. Wimbledon, because after his loss to Roger at RG, he didn´play any grass court matches before Wimbledon and grass was always his worst surface, i mean before Wimbledon i am not sure if he wasn´t 3rd fav after Fedrer-nadal by bookies, and Rome,because he played that long exhausting battle with Andy and next day was playing Rafa at ,,normal,, clay. Actually Madrid wasn´t so surprising for me, because it was clearly fast clay tournament and even in 2009 old Nole push Rafa to his limits at Madrid fast clay courts, so i was expecting that Madrid is 50-50 between him and Rafa, while before Rome everybody was expecting Rafa to finally kick his ass.