Connors had possibly the best backhand of the open era. At the very least, he's in a select group with just a few other players. It was Jimbo's backhand that enabled him to continue to rally effectively against players half his age.
The best two-handers come from players who habitually put a lot of shoulder and torso rotation into the shot (Connors, Djokovic). Too many players, including some of the Americans mentioned in this thread, rely on wristy, arm-oriented strokes that make it very easy to set up for a forehand on the next shot. If you're a savant, you can pull off a backhand stroke like that (Borg). If you're not, you'll produce a Courier/Roddick backhand. Note that when Connors was coaching Roddick, Andy's backhand improved because he paid more attention to footwork and body position. He also developed a decent slice. But by then it was too late to acquire a backhand that was a genuine strength; Roddick's goal was just to prevent it from being too great a liability.