Nagato off course, there are no super powered jinchuuriki to occupy nagato's, or should I say kabuto's concentration. (Kabuto was in control of nagato)
Before yaw say oh but Itachi already one shotted him! Grow a brain, it was a 3-1 battle, Kabuto was focused on Naruto and B, they were his targets and when Itachi snuck up and joined the battle, Kabuto said that now he has three targets and will capture all three of them.
Nagato was busy trying to rip out naruto and B's soul when Itachi got rid of the shared vision and Itachi needed the help of B's TBB and Naruto's FRS combined with his technique just to get rid of chibaku tensei, and only then did he get a chance to 'one shot' (insert sarcasm) nagato.
Now as far as killing each other, they both have a way around the emotality of an edo, Itachi has his sword of totsuka, Nagato has human path, Naraka path's hell demon, and the demonic statue of the outer path's super fast soul devouring dragon that killed hanzo's and danzo's army, plus nagato has chibaku tensei to trap him there. So the issue of imortality is well...not an issue at all.
Genious level = draw, Itachi became an s rank nin at age 11 and defeated oro at that age too. He figured out the weakness of chibaku tensei and he fooled sasuke and almost everyone else for so long, only failing to fool tobi and zetsu, black zetsu and tobi being like the smartest guys in the series. Nagato was an s rank ninja at age 10, this is based on the fact that Kakashi said that to become a jounin one needed to master two nature elements, Nagato mastered, according to jiraiya, all five elements at the age of 10, that is more than twice the 2 that is required for jounin status, so he must have been s rank back then, plus kakuzu used all five too and we know how dangerous someone like that can be. Nagato is also smart, he stayed one step ahead of an experienced jounin like Jiraiya by revealing his bodies only when necessary, hiding the others away and slowly overwhelming him overtime, in fact, his whole pain bodies and shared vision thing is an amazing jutsu thing is an amazing way to make provision for incapacitation. He is also tricked Jiraiya by intentionally falling for his trap with the three paths, knowing that he had already summoned the other three should the others get in trouble and would be able to resurrect them anyway. His ability to make instant analysis of situations and making counter measures is almost unmatched, like when he quickly deduced the strengths and weaknesses of konoha's barrier and then quickly came up with an infiltration plan by sending in only the animal path and then summoning the others from insided, fooling the barrrier into detecting only one interloper when there was actually seven. Animal path was also quickly able to analyze Jiraiya's detection barrier instantly and adjust his strategy to render the barrier ALMOST useless. Basically, they are both smart both on and off the field and are brilliant analysts and strategists, they are also wise and understand a lot of things about life and hardship, although both have chosen very dark paths, ridiculous paths even, seriously, killing your own family no matter what the reason is pure evil and stupid, and trying to destroy the world for peace is also quite stupid and evil, but they are nonetheless equal in genious level.
Amateresu < preta paths absorbtion, deva path's shinra tensei
Sasuno'o < human path's soul removal, naraka path's hell demon king, and outer paths demonic statue and soul remover dragon, might also be weak against bansho tenin, gaara used sand to seperate madara from his sasuno'o, bansho tenin can also pull Itachi out. Sasuno'o, being a jutsu could also be absorbed by preta path, as there is no jutsu without chakra, or he can outright absorb the whole thing, as he was able to absorb the oil from jiraiya's combo jutsu.
Yata Mirror and Sword of Totsuka < most of what was said about sasuno'o, these weapons are spiritual weapons, and anything spiritual can be taken by human path, hell king/demon, and definately by outer path, which is the bridge between the afterlife (pure world) and the real life(impure world) or spiritual world and physical world. Also Itachi seems to need chakra to maintain the weapons as well, in fact from what we have seen, all the weapons of the sage of the six paths require A LOT of chakra to use, preta could pose problems.
Tsukiyomi < rinnegan and Uzumaki heritage plus genjutsu barrier. Itachi once said to Kakashi that only a person of his blood or heritage who has the sharingan can brake his tsukiyomi, and since Kakashi was not an uchiha, he failed to break it. But Nagato is an Uzumaki, and both Uchiha and Uzumaki are descendent from the sage, so they are of the same heritage, plus rinnegan is not only a stronger doujutsu then the sharingan, but it is the ultimate level of sharingan mastery based on what we saw and heard from real madara, so then, this makes nagato fit the profile of the people who can break Tsukiyomi, based on Itachi's own words, though he was a top class liar, Itachi didn't have a reason or motive to lie in this case. Nagato also seems to specialize with barriers and more specifically, genjutsu barriers, he had these genjutsu barriers placed on his subordinates to prevent interogation, so it is possible that he has an even stronger one on himself. We have also never seen him get caught in a genjutsu, just three of his bodies, but not the real him, besides, those three paths could have broken the frog song if they wanted too (I have good reasons for saying this but it is irrelevant to the current topic). In any case, Nagato trumps tsukiyomi too.
Itachi's Water and Fire techs are useless against someone who knows all five.
Yasaka Magatama won't work for obvious reasons, shinra tensei or chakra absorbtion.
Itachi will die, summon spam for shared vision and distraction, summon asura limbs for power and mobility/speed, and destroy him for reason's mentioned above, Nagato rules!
"Itachi, I am going to make sure you feel pain, know pain, and understand pain!" Said Nagato.
"SHINRA TENSEI!"
