Short answer: probably Rosewall
Long answer: This is in one sense a really difficult question to answer. For a start, what do you understand by 'backhand'? The general sense of the answers I have read is an attacking or point-winning shot.But what about a backhand as a rallying shot, to keep you in a rally, or a setting-up shot to manoeuvre your opponent into offering an easy ball to then hit a follow-up winner (baseline or volley). In that latter context Graf would be the outstanding example in recent years (straying away from the men for a moment). And what about the overhead backhand (Federer has a great one)?
Then what about the influence of technology. Modern players can hit shots that older players could only dream about, especially from the baseline. On the other hand the same technology has driven them to the baseline, so they cannot hit the touch and finesse shots at the net that older players could. You can still argue that technology trumps everything, and that by definition the modern player has to be better than players of the past. I don't think that sort of argument is of any real value. The analogy would be to argue that any high school student is greater than Newton because they can carry out calculations that would would have been impossible to him. Or any amateur painter is greater than those from the Middle Ages because we now know how to create perspective, and have acrylic paints which allow a much greater range of colours. I don't think anyone would buy those sort of arguments.
Given all these uncertainties I think the only way to resolve the question is to ask 'which of the great achieving players was considered in their own playing time to have the most outstanding backhand of that time?', and the answer is Rosewall. I don't think any other great player was so well known for their backhand. The OP (pc1) makes a case for Laver because of the greater versatility of his backhand. But if your 'A' shot is so good that it does the job, there is no need for a 'B' or 'C' or 'D' shot. It is great to have them, but it does not mean that overall they can achieve more than a pre-eminent 'A' shot. The only thing that matters is the final result. And Laver had other shots that were superior to Rosewall, so it must mean that overall Rosewall's backhand was the better shot.
One way to resolve this might be to look at the numbers of winners from backhands by different players. But even statistics have their dangers. Taking Rosewall as an example, the known danger from his backhand lead his opponents to favour attacking his forehand, giving him more winners from that side than you might expect given the quality of his backhand. You see the same effect in reverse with players like Lendl, Sampras and Federer with great forehands, where opponents went more often to their backhand, so the raw statistics on backhand/forehand winners does not really reflect the difference between the two.
[PS As an aside I just want to comment on the danger of statistics in another context. I have seen statistics on serving where Karlovic and Isner (and players like Arthur) have some great statistics but....they rarely if ever get/got to the sharp end of major tournaments. Servers like Sampras and Federer on the other hand did get to the sharp end very often where, by definition, they would be meeting the great returners. So their statistics will be hit by spending more time serving against great returners. The way to normalise this is to compare statistics against say, top 10 players only, or top 5, or top3. But that is a lot of work! Anyway I digress...]
I am a little puzzled by the inclusion of Edberg. I assume that is on the basis of his backhand volley. My instinct is to say that I doubt it was special in the grand scheme of things since Laver, Rosewall, Sedgman etc all had rock-solid volleys because it was second nature. However given the difficulty of volleying with more modern rackets I can see why he might be in the discussion.
I do think there is on glaring name missing - Borg. His backhand may not have been the aggressive beast some others have been, but that was not it's nature. It was death by a thousand cuts, or death by the rapier, eye-of-a-needle shot after drawing the opponent in to the net. He may have had a more-than-adequate serve, and he may have volleyed more than people think, but it was that backhand that was noticed when he played. So he must be in the discussion.
And finally I want to add two names - Nalbandian (mentioned in the OP), and Safin. They are interesting because they tend to be rejected from discussion because the players did not end up in the highest echelons. But are we discussion the pure shots, or the player plus the shot? Safin's backhand was a weapon that never broke down, and at its most powerful was a thunderous winner. Suppose we put Connors head into Safin's body, what might that shot have achieved? Similarly if we put the determination and will-to-win head of Rosewall, or Laver, into Nalbadian, what might his wonderfully controlled backhand have achieved.
So, is it the shot, or the player+shot, that we are discussing. And which shot? [And I am glad Ashe's name was brought into the discussion - the greatest gambling backhand. If Gasquet had that attitude....]
