I'm only referring to the best one handed backhand.
Kuerten or Henin definitely have the best backhands of all time. Kuerten can hit far more topspin backhands than Federer. He has a more extreme grip and takes his racquet back and up first so he is prepared to hit the high balls. Kuerten's backhand is far better suited for the modern game than Federer's. Federer has a great backhand, but with his continental grip he will never be able to hit as many topspin backhands like Kuerten. The continental grip requires being early on the ball, you can't let the ball get up on you like you can with the extreme backhand grip. Just watch clips of Kuerten and you will notice he hits far more topspin backhands and less slices than Federer. Henin, uses the same grip as Kuerten and has the same characteristics. I haven't looked into Gasquet's too much, but I like the way it looks. If someone were to argue that his were the best, I might agree because he has some of the same characteristics as Kuerten. He takes his racquet straight up on the backswing which as stated before allows for an easier swing path on the high balls. Thats my two cents worth.