Hard to choose between the 2013 RG semifinal and the USO final that year. It would've been a wonderful feather in Djokovic's cap to defeat a prime level, lights out in the 5th set Nadal on his turf and probably would've prevented the subsequent defeats later on that summer. On the other hand, losing for the second time in three finals on his favourite surface, especially the manner in which it happened, was an absolute sickener and made me seriously question if a tennis God truly does exist. So yeah, if you put a gun to my head I'd probably go with the USO final.
Having said all this, I do believe everything happens for a reason and the chief reason Novak hired Becker in the first place was due to losing that match so if he had won it who knows, there might be no Boris on the scene and the rampage we've seen from the Earth's mightiest warrior these past two years might never have materialised.
The other good thing to come out of that final in NYC is that it put a rocket under Novak in all the matches he's played against Nadal on hard courts ever since. Like I said the other day, there's nothing like a good spanking every now and then and 11 wins in their last 12 meetings certainly attests to that!