At this point, I see it more as a race to collect the Sage's powers than _____ is the Sage's reincarnation. Naruto has Sage Mode, Uzumaki genes, and the Kyuubi (a piece of the Jyuubi). Nagato had the Rinnegan and Uzumaki genes. Obito and Madara have Uchiha genes and the Rinnegan and now the Jyuubi. Sasuke isn't in the race at the moment, but he will be as soon as he realizes what the goal is (since he hates being behind Naruto in anything)

Obito, Madara, Naruto, Sasuke, Nagato. Was there a sixth character who was in the running? Itachi was capable, but wasn't interested in seeing the end of the ninja world in those terms (i.e., in terms of being an unstoppable force of nature that changes everything). I suppose Kabuto might count, since he got himself his own Sage mode, and might have Senju genetics in him through Orochimaru (maybe, I am speculating, but the resemblance between young shodaime and Orochimaru is pretty uncanny). Certainly, these six have chosen very different paths to sagedom.