I think, for me, it comes down to Connors, Rosewall and Agassi but I wouldn't say one was necessarily greater than the other. However, if forced to, I'd choose Agassi for the simple reason that he was slightly more dangerous off both forehand or backhand (ignore power completely, look at effectiveness of shot). However, given that the other two players won significantly more in their careers than Agassi, you can see that any distinction is very minor.
Agassi's return was the most potent, in terms of sheer pace, but, of course, he had the advantage of modern equipment so he could afford to be more offensive than the other two as his margin for error was greater (due to the technology). Connors' return was spectacular but we know he had a weakness on the forehand and he had difficulty when he wasn't being fed pace. Of course, very few players could exploit that on any surface other than red clay. Rosewall was weaker off the forehand than backhand but, given that his backhand is one of the best shots in the history of the game, that isn't so surprising. The choice for his opponents was to hit to a very strong forehand or an unbelievable backhand. They usually chose the forehand but it really was the difference between sudden death and six months in hospital: painful either way. Agassi is similar but with considerably more power and possibly more technically sound off the forehand than Rosewall and significantly moreso than Connors. Power and the ability to hit cold winners really is just a product of technology and really has nothing to do with the quality of a player's return game. Give Connors power and he could hit winners on either side. Rosewall had less power but hit winners left, right and centre due to timing and placement. End of the day, they could all hit winners, just in slightly different ways. Agassi and Rosewall could live with or without pace, as their records on European clay illustrate. Rosewall had the better record on that surface so he probably gets the edge in point construction off the return.
End of the day, I think those three are the very best we've seen and the only thing that could split them is your determination.