There have been so many great backs....but each generation had their own great back person...Columbo/Arnold...for whatever reason, there was a black body builder with great arms...and also had an amazing back...I just can't remember his name...Robbie?
Haney for his day had a very good back...might not have been the best for his generation...but you get the point. They just keep getting freakier.
I admit I am not a Kai fan...but...his back could become a new standard. The downside to him, from my POV, is there is no waist or lower lat definition that Yates carried.
To me, at this point, it would be Yates/Coleman, with each having great backs, though different in over detail.