Rengoku is most likely stronger. Tanjiro says in #131 that Rengoku was “stronger” than anyone else. This would include Hashira he has seen in combat like Tengen, Mitsuri and Muichiro. Would there be any different if Rengoku and Tengen also fought against Muzan?
6) Kyojuro Rengoku
While he was close to beating Akaza, the Upper Moon 3 didn't really reveal his true potential and managed to escape into the forest. He is definitely stronger than Mitsuri since he was her mentor when she entered the organization.
6 Mitsuri Kanroji, The Love Hashira
Swordsmith Village showed just how strong this Hashira really is, wielding a whip-like nichirin blade that looked pretty unique and gave Hantengu's toughest form a run for his money.
3 Hashiras that can beat Rengoku in Demon Slayer
Gyomei is considered to be the strongest Hashira in the series. His contribution was monumental in the battle against Kokushibo and Muzan.
Demon Slayer's Love hashira Mitsuri Kanroji is a lot tougher than she looks. She could easily take down anime swordsmen like Kenshin Himura or Kirito. Mitsuri Kanroji's bubbly personality and unique look made her a fan-favorite character soon after she was introduced in Demon Slayer.
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.
Demon Slayer: Kimetsu no Yaiba's Mitsuri Kanroji is another Hashira who effectively established their strength level in their proper introduction. In the “Swordsmith Village” arc, Mitsuri held her own against the Upper Rank 4 Kizuki, Hantengu, and while she needed help to win, it was still nonetheless impressive.
After awakening his demon slayer mark, Giyu greatly overpowers Akaza. However, he is able to keep up with the fight only because of Tanjiro. If Giyu had soloed against Akaza, he would have died.
Rengoku's Unwavering Will Was His Greatest Weakness
Rengoku sees any mission through to the very end, even at the cost of his life. While his dedication can be viewed as admirable, it is also reckless. During the final battle against Akaza, Rengoku refused to relent even though the demon overpowered him.
Suddenly, Mitsuri Kanroji grabs Muzan's left arm and rips it clean off his body in anger sacrificing her own arms in the process.
But there is one who is the friendliest of them all, and that is Kiyogiro Rangoku, the Flame Hashira. Among the pillars, there is not a single negative thing revolving Rangoku. Such an honest guy, and his character influences others positively. In Demon Slayer, Liu and Rangoku are not shown to have any interactions.
While Mitsuri can withstand an attack from an Upper Rank, given the unique structure of her body, she wouldn't outlast Akaza, especially considering she wasn't able to defeat Upper Rank 4, Hantengu, on her own.
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. In other words, she's incredibly strong and powerful, and tough with plenty of endurance as well.
Giyu is one of the legendary Hashira, and he's brought down demons far more powerful than Nezuko. With his level of skill and impressive agility, he'd easily defeat Tanjiro's little sister. In fact, when Giyu first meets Tanjiro and Nezuko, he almost does kill the latter.
If Zenitsu fought Akaza with his plethora of Thunder Breathing techniques, the latter wouldn't have the time to regenerate his limbs.
Not to mention that no one has been able to kill an Upper Moon in over a hundred years. Uzui and Tanjiro's group breaks the cycle of tragedy when they defeat the Upper Six, Gyutaro. However, it takes one Hashira and several more demon slayers to take down the weakest of the Upper Moon.
Gyomei Himejima, the Stone Hashira, is the strongest Hashira in the Demon Slayer corps in terms of strength and swordsmanship. He is the only person who uses spiked flails and an axe instead of standard swords to kill demons.
3. Kyojuro Rengoku – The Flame Hashira. Rengoku is a key player in the Mugen Train Arc, culminating in an amazing fight between him and the Upper Two demon Akaza. Despite Akaza being so powerful, Rengoku is able to hold his own for quite some time before his tragic death.
Featured character Tengen Uzui has also been a heavily debated character as a result, specifically in regards to power scaling. It seems as though some fans feel as though Uzui is the weakest of the Hashira in the series. While he's certainly not the strongest, he's far from the weakest as well.
Something I noticed with Iguro and Mitsuri is that in the final battle Iguro loses his eyes and Mitsuri loses her arms which Iguro's heterochromia and Mitsuri's freakish strength are major causes for their respective tragedies and even though Mitsuri cannot hold him and Iguro cannot see her they proclaim their love for ...
2) Mitsuri Kanroji
Despite being slower with her movements out of all the Hashiras, her techniques are noted to be faster than the Sound Hashira. She can defeat Zenitsu effortlessly and can counter his speed by only unleashing her Second Form of Love Breathing: Cat-Legged Wings of Love.
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.