1) Inosuke Hashibira Although Inosuke possesses immense strength and endurance, he's no match for Zenitsu's insane speed and his Thunder Breathing.
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.
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. In fact, he became one of the very few to even invent a new form of Thunder Breathing. However, his dream of becoming a Hashira was never realized.
Far from it. Tanjiro and the rest of the hashira could probably easily solo him at his peak. His largest achievement is killing Kaigaku, who was probably around as strong as Daki since he hadn't become used to his transformation.
2) Inosuke
His performance against Doma definitely puts him on Hashira's level. However, his battle against Shinobu could go either way.
Which Hashira can beat Nezuko? The Wind Hashira is one of the strongest Hashiras in the Demon Slayer series. He played a significant role in defeating Kokushibo and was able to inflict damage on Muzan. Sanemi's sheer power and speed will be able to overwhelm Nezuko even in her final form.
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.
True Power
While unconscious, Zenitsu is a force to be reckoned with, but his actual power is discovered when he is awake. While awake, Zenitsu was able to match the upper rank six, Kaigaku. He was also able to keep up with the Demon King, though he was in a weakened state during this battle.
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.
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 blitzing Upper Rank Six, Kaigaku. Immense Speed & Reflexes: Zenitsu is extremely fast, being able to easily move faster than even the eye can see.
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.
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.
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.
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.
3) Zenitsu Agatsuma
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.
Intimidation alone would allow Giyu to beat Zenitsu in a fight, but it's also obvious that Giyu's combat skills would allow him to crush the other boy based on physical ability alone. Zenitsu has a long way to go before he reaches Giyu's level.
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.
Exploring Zenitsu's fighting abilities in Demon Slayer
Zenitsu has perfected the first form only and is seen performing it only when he's unconscious. But the manga shows that he can fight without fainting or going unconscious.
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.
This wasn't the case with Zenitsu, whose mastery of Thunderclap and Flash is clear for everyone to see. This mastery allowed Zenitsu to form a Seventh Form of Thunder Breathing that is unique only to him: Honoikazuchi no Kami.
Zenitsu may be faint-hearted and tend to pass out when he gets too afraid - thus placing him into a bizarre kind of sleep - but he actually has stellar Nichirin Blade skills and breathing techniques. Fortunately for him, being unconscious is the key to activating his combat skills.
At this point in the manga, the Hashira had yet to gain their marks, and were later required to team up in order to kill the Upper Ranks. Fighting one-on-one against Akaza for very long would have been nearly impossible for any Hashira, but some could have lasted until sunrise.
8/13 Shinobu Kocho, The Insect Hashira
That being said, she's arguably the weakest Hashira when it comes to pure strength.
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.