Grigor Dimitrov and Stan Wawrinka have the best one-handed backhands in the game today IMO. So tough to make that work today, because with slower surfaces and a premium on defense, it is not as easy to end points as quickly as it was in previous eras. That single wing is not as physically stout as the forehand or two-handed backhand. Guys like Nadal will bludgeon that side until you tire and start shanking the ball.
Ken Rosewall has the greatest one-handed backhand ever IMO. What set him apart was the way he texture the ball with his slice. It was crazy to watch him move the ball around.