All time best:
1)
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Flawless offense and flawless defense plus flawless mobility and versatility with all Kamui powers. Smartest fighter in the manga with all Kamui hax and Rikudo chakra and Perfect Susanoo raw power at his disposal. Kamui Raikiri that blitzed even Kaguya. He alone could fight the most powerful version of Kaguya much better than Naruto and Sasuke together could previously do against her. No wonder his powers were just temporary.
2)
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Incredible speed and raw power, could hold his own against Kaguya. Kurama's Bijuu Mode plus Senjutsu plus Rikudo Senjutsu due to chakra from Hagoromo and the other Bijuu as well. As an adult he even gained the other half of Kurama chakra.
3)
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Almost Naruto's equal, but he is not on Naruto's level, Naruto could outperform him clearly against Kaguya and could draw against him even fighting without killing intent, while Sasuke fought him with killing intent and trying every possible resource to defeat him.
4)
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God level taijutsu, speed and power output. The fact that he could blitz and ragdoll Juubi jinchuriki Madara all around says everything.
5)
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Not on God Tiers level, absolutely, he can't compete with them, but he is the most powerful character out of them. Wonderful base stats and his Mokuton is astonishing, and all of that is enhanced by SM. And Senpo: Shinsuusenju is out of the world.
Not counting Obito and Madara since they defected Konoha one way or another.
Current best:
1)
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Obvious.
2)
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Obvious.
...
3)
Post War No Sharingan Kakashi
He lost the Sharingan with all his advantages (precognition, copying, genjutsu) and the MS with Kamui hax, but he is still a greatly well rounded ninja: fast, skilled in CQC, hella smart and versatile, with a great ninjutsu arsenal. Without the Sharingan and the Mangekyo draining his chakra, with his War Arc feats, he now can spam his ninjutsu arsenal. And he created
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, which is
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. That shit basically ripped the sky in half and provoked a storm and greatly impressed Onoki that was put in awe by
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.
4)
SM Kabuto
He lost Orochimaru's powers, but he is still a very resourceful and power opponent with what he retained. But probably he retired from being a shinobi.
5)
Adult Rock Lee
Not on Gai's level, even without accounting 8th Gate, but his speed, power and taijutsu are over the roof. Great feats in The Last.
Also, it's interesting to say, like other people already addressed, that Gai didn't retire as a shinobi. He is crippled without have the use of his right leg after the end of the War, but in Kakashi's novel he shows he can still fight greatly. Also his arms are okay, so he can still use Asa Kujaku and Hiru Tora. And theoretically he could still open the 8th Gate and spam Sekizo.
Then Sakura, Choji and Shikamaru are honorable mentions
