Part of the reason he has 2 WTTC titles and an Olympic Gold in singles is that ZJK has a pretty darn good backhand. He can control a point and keep an opponent on his backhand because of how good his is. If there is a backhand to backhand rally and the player tries to turn to the forehand, they are in trouble against ZJK. I would say ZJK has the best and there are a few close behind him. Fan Zhendong, at some point will probably have the best backhand. But it is true, Dima's backhand is awesome. Pretty close.
Back in the day Schlager's backhand was pretty darn solid. And there is no need to mention Kreanga's. His is still pretty darn amazing. When he was in his prime, that was, perhaps, the most powerful backhand ever.