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.
Tanjiro Is Strong, and Stronger than Zenitsu as he uses Sun Breathing.
Unconscious Combat: Zenitsu has the ability to fight when he sleeps, which allows him to awaken his true strength. While awake, his nerves and fear cause him to freeze up so he cannot move. When his life is in danger, his nerves and fear grow so strong that he passes out.
1) Inosuke Hashibira
Although Inosuke possesses immense strength and endurance, he's no match for Zenitsu's insane speed and his Thunder Breathing.
Zenitsu has some combination of RBD, narcolepsy, sleepwalking, and lucid dreaming that allows him to behave as if he were awake while asleep. Thanks to his extensive training and superhuman hearing, he's able to detect and fight demons in this state.
When facing Kaigaku during the Infinity Castle arc, Zenitsu debuts his brand new exclusive Seventh Form to Thunder Breathing. With this, Zenitsu finally has more than one tool in his arsenal and has overcome his own personal doubts and fears.
Why Zenitsu could not become a Hashira in Demon Slayer. Fans have wondered whether Zenitsu ever got to realize his dream of becoming a Hashira at all. Unfortunately, the answer to this question would be, no, he did not. Zenitsu, along with Tanjiro and Inosuke got way stronger than average corps members with training.
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.
3) Zenitsu Agatsuma
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.
Zenitsu Agatsuma vs Kaigaku is a battle that took place in the Infinity Castle. It focuses on the battle between Zenitsu Agatsuma against Upper Rank Six Kaigaku.
Zenitsu is a skilled and able member of the organization but gets easily scared when facing demons. But as the series continues, we watch him grow as he eventually overcomes his fear of demons and fights them head-on.
So in a nutshell, Zenitsu wasn't able to master the other five forms, but later in the Infinity Castle Arc, during the battle with Kaigaku, Upper-Rank Six Demon, he created a Seventh Form: Flaming Thunder God.
Zenitsu speed is nearly that of light. To downgrade it, it is still invisible to naked eye, i.e. more than 51 Mach. Zenitsu speed with his sword skills just blitz Kaminari.
Thunder Breathing has 6 standard forms in total, with the first form said to be the foundation for all the other forms. However, Zenitsu Agatsuma developed a Seventh Form in order to stand on equal ground with Kaigaku, his former senior disciple under Jigoro who had become the Upper Rank Six of the Twelve Kizuki.
He's mastered the first form to utter perfection making it deadly and he even made a godspeed version. I don't think he could take Kokushibo, but with enough training and a mark he would obliterate the other upper moons. Doma go bye bye.
If he could, Rengoku would win simply because Zenitsu cannot perform Godspeed more than twice. Since Zenitsu only knows the first form, Rengoku would know what to expect and accordingly counter his moves, taking him down in the end.
Undoubtedly, Muzan is the fastest and strongest demon in Demon Slayer.
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.
After the deaths of Daki and Gyutaro, Kaigaku was promoted to become Upper Moon Six of the Twelve Demon Moons due to his power and confronted Zenitsu as a rematch he always desired.
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.
Zenitsu is at least close to Hashira level by the end and Godspeed has limited uses because it disables his legs after those uses. Zenitsu's faster but if this was an actual race or marathon, Shinobu would beat him.
Of course, Zenitsu is only the master of the first form, but it goes without saying that the other techniques of Thunder Breathing will also be awe-inspiring to witness in Demon Slayer. Given how powerful the first form already is, the wait to see the rest of Thunder Breathing's arsenal is positively agonizing.
Despite her petite appearance, Mitsuri Kanroji has a special muscular composition that makes her muscles eight times denser than an ordinary human's. She is also more flexible than the other Hashira. These two strengths balance out her being the slowest runner out of all the Hashira.
13/15 Zenitsu Agatsuma Defeated An Upper Moon On His Own
He falls asleep when he crumbles under the weight of his own fear, and his body fights on instinct alone, utilizing his Thunder Breathing Style. He proved that he can be a capable fighter even while awake, defeating Upper Moon 6 on his own.
Yes, he faced one of the requirements to become one!!! He defeated on of the 12 Kizukis by himself and is at the rank of Kinoe! But, of course, the Demons Slayer Corps disbanded, so I guess he doesn't have a chance to be one, but either way I'd say yes!