Upper Moon 1: He won't be able to defeat him alone, if it wasnt for genya trapping kokushibo, tokito inflicting damage, pain, and holding upper moon1 down with his red blade, and sanemi helping gyomei and sharing the bright red color to gyomei's weapon and also helping remove the head.
In the Infinity Arc, Gyomei was capable of overwhelming Kokushibo, and the latter even acknowledged the fact that he has fought numerous swordsmen in his lifetime but no one was as strong as Gyomei.
Even the Hashiras who are filled with pride, acknowledged his skill and strength. As a child he was able to save a child from a demon by physically beating it for hours. He, along with Sanemi Shinazugawa were able to beat the Upper Moon 1, Kokushibo.
In most cases, yes, a Hashira alone usually can't defeat an Upper Moon thanks to the latter's crazy regeneration, they would need at least supports from other demon slayers.
The Hashira are weaker than the Upper Moons. It has already been stated that the Hashira has killed multiple lower moons, while Upper Moons have killed and eaten many Hashira.
Fortunately, Zenitsu was able to single handedly beat the Upper Moon 6. Not only did he defeat the demon, he also created a seventh form called the Flaming Thunder God. These are the demons who have been given the title of Upper Moon 6 in Demon Slayer.
Kokushibo is reserved, silent, and aloof, maintaining an aura of unnerving tranquility and mystery that compliments his position as Upper Rank One.
5) Yoriichi
Case in point: he was the only one who could single-handedly take on Muzan and drive him to a corner. Yoriichi would be able to defeat every single Hashira on this list without breaking a sweat. Such was his reputation and skill in swordsmanship.
At the end of the day, none of the Hashira could've defeated Akaza – even though a few of them could've succeeded in decapitating him.
Sound Hashira
He is the fastest Hashira, and the second physically strongest. Obviously, then, Tengen Uzui is rather proud. He is demanding, fussy, boastful, and prone to selfishness. But he is also ultimately kind and caring, with great love for his three wives as well as the other Hashira.
At the end of the day, none of the Hashira could've defeated Akaza – even though a few of them could've succeeded in decapitating him. Akaza proved himself to be too strong to be dragged into sunlight, meaning there'd be no way for any of the Hashira to kill him.
Doma can't stand a chance against Gyomei's monstrous strength and endurance.
With that said, all Hashira in Demon Slayer have one glaring flaw: they're only human. As such, they're not infallible and are certainly not tougher than their demon enemies. This results in some rather lopsided matches and emotionally-charged oversights on their part.
Tanjirō's resemblance to Yoriichi (like from Muzan's perspective), would trigger his memories and cause him to accept death. AND since its against Kokushibō and not Muzan, there's not poison that'll mess him up. So I would say Tanjiro would win.
4) Kokushibo
The Upper Moon 1 is a swordsman and is capable of causing a ton of destruction from any range. Nezuko will have to give her all to just keep up with someone like him. Kokushibo would easily be able to defeat her in a 1v1 combat.
Tanjiro would have the necessary speed from both his breathing techniques and supreme territory, while using the see through world to match up with Kokushibo breathing technique attacks. Tanjiro would also pull a Yorrichi and cut off Kokushibo's head in a blink of a eye and or overwhelm him.
Yoriichi was born with the Demon Slayer mark and prowess in swordsmanship. His powers were beyond comparison as he defeated Muzan once and had him on the verge of death until he managed to escape.
Giyu Tomioka and Sanemi Shinazugawa attend as the final surviving members of the Hashira, and together they they get a confirmation that not only are the demons all eradicated, but that the Demon Slayer Corps will be disbanding.
Mitsuri Kanroji is another Hashira who Tanjiro could definitely beat in Demon Slayer. Mitsuri's main trick up her sleeve is her muscle composition, which makes her muscles eight times denser than that of a normal human's. In other words, she's incredibly strong and powerful, and tough with plenty of endurance as well.
Three Hashiras capable of besting Akaza in Demon Slayer.
But if we are talking about the human form, then it's a big fat NO, because Tanjiro has just mastered the Sun Breathing whereas Yoriichi is the inventor of it, also he's way more powerful and holds an unfathomable battle intellect, larger than any Demon Slayer in history, so in conclusion, no one is remotely close to ...
Tengen is one of the only known Demon Slayer and Hashira to retire due to being permanently disabled instead of death or age, the other being Jigoro Kuwajima.
That being said, Tanjiro's rank is unknown, and his rank was last revealed in the Entertainment District arc. In that arc, they were at Kanoe, which is fourth from the bottom. Since Kiriya Ubuyashiki announced the disbanding of the organization, Tanjiro doesn't get the chance to become a Hashira.
Zenitsu blitzing Upper Rank Six, Kaigaku. Immense Speed & Reflexes: Zenitsu is extremely fast, being able to easily move faster than even the eye can see.
Kokushibo (黑死牟) is the secondary antagonist of the manga/anime series Demon Slayer: Kimetsu no Yaiba. He is Upper Rank One of the Twelve Kizuki, and the archenemy of his younger twin brother, Yoriichi Tsugikuni.