Zenitsu, with his insane strength and lightning speed, can eliminate every Upper-Rank Demon in 1v1 except Upper Moon 1, and Muzan because in terms of strength they are more formidable than him.
Zenetsu will also engage in a battle against the demon king, Muzaan Kibutsuji. His fighting skills develop with each battle he fights, and later on, Zenitsu becomes a Hashira. He does not just fight Muzan, but he is actually able to severely injure him with his attacks.
1) Tanjiro Kamado
Although he and Zenitsu along with Inosuke, started training together, he excelled both in the matters of strength and swordsmanship. Tanjiro with his Sun Breathing is more powerful than Zenitsu.
Overall, Zenitsu is an extraordinary powerful Demon Slayer capable of singlehandedly bringing down an Upper Rank, which said to be able to overpower 5 Hashira, in 10 minutes, albeit he would have beheaded Kaigaku in a heartbeat if it wasn't for his feelings of melancholy and pity.
Alucard from Hellsing Ultimate is not your regular bloodsucking vampire. He's one of the anime characters who could potentially defeat Muzan Kibutsuji with a single blow. Alucard is known for his cold, calculating personality, which is matched only by his incredible powers.
Zenitsu, with his insane strength and lightning speed, can eliminate every Upper-Rank Demon in 1v1 except Upper Moon 1, and Muzan because in terms of strength they are more formidable than him.
The only thing he's weak to is the sun. That is why he turned other humans into demons and spread his blood so that one of them might find a solution against the sunlight, so he can become truly perfect.
slow ?! , Deku was able to dodge and hit in a split second. And yet he is clearly faster than Zenitsu .
Demon Slayers are known for their strength and bravery in battle, but not Zenitsu. Despite his cowardice, he's one of the strongest in the series.
His immense stamina and will, makes him a powerful opponent as he is able to go long periods of time in battle without giving up. Just like his comrade Tanjiro, Zenitsu also refuses to give up in the heat of a battle and was able to battle with extremely stronger demons such as Kaigaku and Muzan.
But, comparing the two characters in their best forms, Zenitsu might be just a tad bit faster owing to his Godspeed move.
2) Tanjiro Kamado
With Sun Breathing, which is regarded as the paramount of all Breathing Styles, Zenitsu will have a hard time subduing Tanjiro and will be defeated by the latter in no time.
Thunder Clap and Flash—Godspeed is the most powerful of Zenitsu's derivations of the Thunder Breathing first-form technique.
The warrior Muzan feared was a swordsman named Yoriichi Tsugikuni. Yoriichi was the only slayer before Tanjiro who came close to killing Muzan, and the flashback of their fight occurred in Chapter 187.
Yoriichi Tsugikuni, the first swordsmen to ever use Breathing Styles, once nearly killed Muzan with Sun-Breathing, and Muzan has been both physically and psychologically scarred ever since.
1 Muzan (Demon Slayer)
Without that antidote, it would have been impossible to kill him. So despite all her strength and humanity, Nezuko would have easily lost to Muzan.
Given how talented Tanjiro is, it's likely he'd defeat Zenitsu, even if the latter could fall asleep and call on his powers. Tanjiro's demonstrated his ability to take down more powerful demons and adapt to training on more than one occasion, so it just makes sense.
After Muzan was defeated, all the demons he created also perished immediately, so there was no point of a Demon Slayer corps anymore. As a result, Zenitsu did not get to become a Hashira even after becoming strong enough to bie one. Demon Slayer is one of the most popular anime series going on right now.
Killua Zoldyck's "Godspeed" allows him to reach a velocity comparable to Zenitsu's own. He has many advantages that would allow him to dominate his demon-slaying counterpart, particularly through differentials of experience.
Tengen Uzui
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.
Since Muzan favored Gyutaro so much, he chose to incorporate the latter's sister Daki as well. The demon king believed that she was the only tool that could be used to control Gyutaro.
It is revealed that Tamayo's drug had an aging potion mixed in along with the Demon antidote. Due to this, Muzan has aged 9,000 years within the span of three hours.
The curse prevents Muzan's name from being spoken aloud, and any who do will meet a horrible demise. One demon, Susamaru, faced this reality when battling Tamayo. Susamaru accidentally shot Muzan's name aloud, attempting to defend his honor with her mind clouded.