Its simple Nagato who Naturaly awoke to the Rennigan, even if madara Transplanted his eyes into Nagato at a young age, which i don't believe considering there is a time of healing between the implanting of the Rennigan, there is no way Nagato wouldn't have knowledge of it anyway, Nagato's eyes weren't Always Rennigan Eyes he awoke to them when he witnessed his parents death and lived and learned. Obito Watched as Nagato Learned and he Learned from Nagato's Use of the Rennigan. Thus Obito was able to use the Jutsu/Dojutsu so quickly after the Transplant of already active Rennigan. its not that Obito was the better user, but he didn't have to go through everything Nagato did to learn the Techniques, he simply watched Nagato, and he may have even used his Sharingan to remember it. So if you think about it, Obito was lucky he got to witness the power of the Rennigan before obtaining it or he wouldn't have been able to use it for shit. Madara's a diffrent story of crouse, considering he awoke to it Naturally and lived with it learning its techniques over time like Nagato. but of course he knew all of its secrets, and died, then now we see him using it. [also the reason Obito only had one Rennigan it was because he didn't have a large enough Chakara Pool. I think the same goes for Sasuke as well and i mean you'd have to have a ridiculous amount of chakara to use too fully fledged Rennigan.]
