Nalbandian's backhand is still the best on tour even today. Not to mention that one that best measures up to Nadal's forehand even with David far from his best.
That said, Djokovic's is an excellent shot as well. But it's not really that much better than, say, Murray's. The only difference, which is kind of a big win, is that Nole can generate more power off that wing than Murray. In terms of mechanics, angles, etc.. they are not that different.
true but murray still has a better (backhand) slice than djokovic.
overall, given murray's variety in that wing, his backhand > djokovic's backhand.