Naruto and sasuke undoubtly..
I would say Nagato and Itachi with very high/near death difficulty as Itachi is clearly sooooo overpowered its almost unfair. Everyone says Naruto is overpowered, very true, but the guy cant defend against genjutsu to save his life, and that happens to be one of, if not, the strongest weapon in Itachi's arsenal. Naruto's easily neutralised, thus leaving Sasuke. I dont even rate Sasuke that highly tbh. True, hes very powerful, but compared to Itachi, Sasuke still has a long way to go.
Add to that the fact that Itachi is completely healthy, and you have an almost invulnerable fighter, with his ridiculously overpowered Susanoo and his larger chakra reserve, due to him being healthy. In the manga, it is stated that he used medicine for some time to prolong his life, so he can face "Saucegay". A completely healthy 13 yr old Itachi killed off one of the, if not the strongest clan in Konoha, possibly the shinobi world. This guy is on a whole different level to "Saucegay".
People say Naruto's already beat Nagato, but people intentionally forget the fact that Nagato took out ALL the ninjas in Konoha before facing Naruto. The guy turned the whole of Konoha into a giant crater. THAT surely must have taken a lot of chakra up. He fought Kakashi, Tsunade, and ALL ninjas in Konoha. Also, with Katsuyu healing almost everyone, Nagato had to fight many ninjas twice, maybe more. To state, with unshakable belief, that Naruto will "own Nagato", is folly and pretty ignorant. If they were to fight again, it would be a much more even fight, however I still think Naruto will take it, just about. He'll probably be one decent punch away from dying; thats the level of fatigue he'll be feeling after his fight with a fully fit Nagato, imo.
Itachi, as stated above, is on a whole different level to Saucegay. I dont think anyone on here can argue otherwise. Feel free to do so. Even if Naruto beats Nagato, Itachi easily finishes him off. In the end, Itachi remains imo, with all others dead.
For all those who dont understand why people say "Saucegay", its because people dont generally like him, and his name is pronounced "Saaskay", which is similar to "Saucegay". THIS LAST PARAGRAPH IS ONLY FOR PEOPLE WHO DONT UNDERSTAND WHY PEOPLE CALL SASUKE "SAUCEGAY".