It's quite unexpected, but I do feel/think that the person most likely to derive the most benefit from the Rinnegan right now is Naruto. His Uzumaki Stamina/chakra is already the rival of anyone except Hashirama, plus as the Kyuubi's jinchuuriki and also A Sage, he can use the Rinnegan's Izanagi to the max, and not break a sweat.
This is Ironic because Naruto is unlikely to get the Rinnegan naturally, and He Won't/Shouldn't do transplants. The Only way he'd get a Rinnegan is if His Physical/Life force energy was so super-over-the-top, that in addition to whatever meager spiritual power he does posses he still generates/evolves a Rinnegan. Problem is that for this scenario to occur, Naruto must have some lineage back to the Elder Son, and His Life force must approach that of the Younger son. Certainly it would have to exceed Hashirama's IMO.
Currently there is no evidence that So6p was born with Rinnegan, rather it is even suggested that he wasn't, that's why in a legend it is mentioned that it was a gift from the heaven and not an inborn trait. As to the one that dwells within the heavens, the one that shapes the earth, the beginning and the ending, it's Jyuubi (seeing as one of the Bijuu can fly, I won't be surprised if the 10-tails can as well, actually it would make more sense seeing as it's destruction is point-based and not line-based).
At this point in time, Kishi already showed that the most important mutations are results of chakra. To be more exact, the energies are the source of it. Natural Energy has the greatest strength when it comes to structural mutations. Spiritual Energy has the greatest input on quality of mutations, while Physical Energies on their range and depth (ok this on is a tricky one ;P).
The Bijuu chakra, while possessing powerful energies of both in great quantity, proved to be able to easily mutate people (Naruto's face, S-G Brothers), not to mention the one possessed of the ultimate chakra being the Jyubi. The Natural Energies as well have quite power themselves, look at Jyugo, Naruto, Jiraya, Oro, Kabuto and all those frog stone statues as well.
Now, we have So6p, someone who was in contact with the most powerful Bijuu and according to Kabuto, a master of Sage arts as well. Not to mention what his inherited traits could have been. With all those, it would be quite strange for him to have no mutations, unless he had one, a strong and visible one at that, the Rinnegan.
Of course, simply sealing a bijuu inside won't mutate you permanently, as shown by other Jinchuriki including Kurama's previous ones as well. There needs to be at least a direct contact or even the need to eat it. It's entirely possible that So6p was born with good traits, which later were amplified by a peace of Jyubi that he ate, fall to, rained on him or whatever else. In similar matter, Kyubi amplified Naruto's Uzumaki traits in order to heal/regenerate it's host (seeing as it only works for Naruto and no other Jinchuriki, it seems to be right).
On the other hand, if So6p was born with good traits, then mastering Natural Energy might have been enough to awaken a Rinnegan within him, seeing as a proper Sage Mode strengthens each of you traits and even activates your body in an imaginable ways.
I find it hard to accept how an Uzumaki, born with Low to ordinary Spiritual energies could Sire the Elder Brother and The Uchihas. There is always the possibility of mutation I suppose, but the simplest assumption without adding any extraneous ideas is that the SO6P was not an Uzumaki, but was the progenitor of Uchiha, Senju, and Uzumaki alike.
My own take of the Gift from Heaven explanation to the SO6P is that He himself was literilly gifted by the Gods, this could mean anything from he being Born that way (straight from Heaven), to he was hit by lightning and transformed.
The so6p and madara do not achieve rinnegan in the same manner. As progenitor, the sage dictated how one could or would achieve his level of power and ability. Madara in blood, chakra, and spirit is a derivative, therefore he must evolve, or rather "return", through a set path to return his branch powers back to its roots. Madara would need to reach rinnegan through means of transforming his mangekyo and also likely his eternal mangekyo.
that was also one of the things that i mentioned that the SO6P could also have been t he creator of the uzumaki clan. and as for the other part, i would say it depends on your definition of spiritual energies is. my definiton of spiritual energies is having a huge amount of chakra or having a particularly strong chakra or resilient chakra. Naruto's chakra easily fits into this, having a chakra pool atleast equal to that of sasuke, if not greater without taking the kyubi chakra into account(4 times the size of kakashi's chakra pool is a lot), even kabuto said that naruto had incredibly resilient chakra and if he ever mastered it, he would be a monster, so from that perspective, naruto has a very high amount of spiritual energies, atleast the same amount as sasuke if not more.
Tobi who introduced the term spiritual Energies already defined it, and he said the elder brother was gifted with the sages eyes, his powerful chakras and spiritual energies, while the younger was gifted with the sages life force and physical energies.
I like Naruto too, but what you are implying is not in keeping with the manga, at least as I read it. :)
The sharingan is technically the rinnegan, albeit an incomplete piece physically and incomplete aspect in essence. As are the mangekyo and eternal mangekyo; all three different portions of rinnegan and all three characteristic and partly reflective in power to the what their sum should equal.
Izanagi sharingan to izanagi rinnegan for example.
The stages and tiers of the optical doujutsus combined into rinnegan is very similar if not exactly the same as the 9 tailed beasts and the juubi. Perhaps divine and miraculous in origin but transformed and disseminated through creation.
While words such as "evolve" or "mutate" might work when describing an uchiha's journey from sharingan to rinnegan, they do in fact remain misleading and not adequately informative of the true process and occurrence. There are much better words suited to this as you would not say "i mutated my one jigsaw piece into a completed puzzle image".
you may have a point, but i cant help but to fall back on the SO6P who didnt have the sharingan and still developed the rinnegan.