In the current era, returning aggressively isn't as important since people don't come in. Murray can beat most people off the ground anyway, so his best bet is to neutralize the serve. It gave Federer fits at Wimbledon and led to Murray crushing him in the Olympic final. Do you think Djokovic would've given Federer the near bagel that Murray gave him in the second set? Djokovic, even at his best couldn't neutralize Federer's return like that, even if Federer was tired. Murray was doing to Federer, what Federer typically does to Roddick.
That is why I woud consider Murray superior to Djokovic.
Besides, what about Ferrer? Guy clearly has worse groundies than Djokovic, but still is ahead of him in return statistics.