Juubito
I think it's possible we've been underestimating his power and/or overestimating those above him; or at least, I have been personally.
Why? Well, we are told by Minato that Six Paths Madara was (much?) stronger than Six Paths Obito. Which makes sense, he had much more of the Kyuubi's and Gyuuki's chakra.
My original hypothesis was that Naruto was, by demonstration, stronger than Rikudo Madara until the latter absorbed the Shinju. Now that I think about it, though, that doesn't make sense.
Why did the Juubi/Biju exist inside Obito/Madara after the former pulled out the Shinju? It wasn't like Obito was massively weakened afterward, and he didn't have Rinne Sharingan beforehand. Yet absorbing it somehow gave a huge power boost, despite Madara already having the Juubi in him?
What?