Djoker's game on clay on 2011 absolutely blew me away. Defeating Rafa in several clay finals was mind-blowing. I still think had Fed not beaten him in the FO semis that year, he would have defeated Nadal.
So I still choose 2011 over Nole of today through September. He faded after that. He has not faded this year!
2011 Novak had super belief in his ground game, he would have enforced errors but he was relentless.
Baseline wise i dont 2015 Novak would win in any hard court slam.
2015 Nole will beat 2011 Nole, because of his much more improved serve and movement.
Clay is a close call but i will give the edge to 2011. 2011 Rome and Madrid final come to mind and he had simply pulverized Rafa, who then was playing great.
Djokovic 2015 is a more complete/sensible tennis player, but Djokovic 2011 was an absolute monster, and if the two were to face off I have little doubt that the 2011 version would treat the 2015 version something like Stan did at Roland Garros. The ATP in 2015 might be about as bad as I've seen it since the early part of the last decade. It's probably worse. The old guys are starting to fade, and the young guys are not stepping up. It's embarrassing, and Djokovic has been strong enough to capitalize on that. I'll take him trouncing Nadal 6 times in a row on 3 different surfaces in the prime of his life over what he's been doing this year any day.
Backhand at least same if not better
Physical stamina/stregnth better or equal (he came up after beating Murray in 3 hr Rome to beat Rafa next day)
Movement less grinding
So I think 2015 Djokovic will win anywhere except clay where he was beating Nadal in rallies on clay.