At the moment, through feats you can say Nagato.
However I say Tobi. I think Tobi exceeding Nagato's skill with the Outer Path (the chains) is a sign that he'll show more than Nagato could do.
It is possible that Madara showed no Rinnegan and Nagato was rushed, all so Tobi could show the Rinnegan.
Perhaps this stance isn't the best at this juncture because I am giving Tobi the benefit of the doubt. However saying Nagato uses the Rinnegan, or can use the Rinnegan, better than Tobi doesn't sit well, imo.