WARNING: The following contains spoilers from Demon Slayer: Kimetsu no Yaiba the Movie: Mugen Train, now streaming on Funimation and Crunchyroll.
The long-awaited second season of Demon Slayer: Kimetsu no Yaiba is nearly upon us, and if the trailers are any indication, the Hashira-in-action trend will continue. In the Demon Slayer Corps, the Hashira are the best of the best, outrageously more powerful than the average swordsman. As with any group of talented people, comparisons are inevitable, and the anime has hinted at the notion that some Hashira may be more powerful than others.
Despite there being nine Hashira in total, to this point in Demon Slayer, only three of them have shown off their skills in combat. While Tengen Uzui, the Sound Hashira, will clearly be the most prominent of Season 2, his power level is unknown like many others. Let's look at the three Hashira we've seen in battle heading into Season 2 and decide which of them is the most powerful we've seen so far.
Shinobu Kocho: The Insect Hashira
Coming in at number three is Shinobu Kocho, the Insect Hashira. The smallest and physically weakest of the Hashira in Demon Slayer, even beheading a Demon requires a physical strength that is beyond her. This is not to say that she is weak, but as Hashira go, Shinobu is likely toward the bottom of the power hierarchy. She has only used one of her Insect-Breathing forms so far. It was more tranquil and distracting than powerful, but nonetheless got her close enough to deal significant cuts to a powerful Demon.
Shinobu makes up for her lack of physical strength by utilizing deadly poisons, specially designed for killing Demons. A few cuts were all she needed to poison a Demon that had been given power by one of the Twelve Kizuki. Shinobu is also quite nimble, allowing her to effortlessly evade her opponent's strikes. Her greatest value so far has been as a mentor, whose excellent teaching instincts helped Tanjiro, Zenitsu, and Inosuke master Total Concentration Breathing: Constant, a technique used by the Hashira for greater power.
Kyojuro Rengoku: The Flame Hashira
While certainly leagues above Tanjiro and the gang as a Demon Slayer, Rengoku's tragic distinction of being the only Hashira so far to fail to defeat a Demon opponent lands him at number two. Granted, his opponent was Akaza, Upper-Rank Three of the Twelve Kizuki and one of the most powerful Demons on the planet, but a stronger Hashira likely could have defeated him, given how close Rengoku came. It was painfully clear during their battle that Rengoku wasn't that much of a match for Akaza, but it's not always the strongest who triumphs, as Kyojuro nearly bested the Demon in a battle of wills.
Kyojuro's Flame-Breathing was devastatingly powerful against average Demons and even dealt Akaza some severe wounds. Clearly a powerful Hashira, the Demon Slayer Corps is in a lot of trouble if Rengoku was their most powerful warrior. Unfortunately, Rengoku's time was cut short with his death at the hands of Akaza, so it will never truly be known how powerful he was in comparison to the other Hashira.
Giyu Tomioka: The Water Hashira
Giyu Tomioka seems the clear answer for the strongest Hashira seen in Demon Slayer so far, as his Water-Breathing utterly humiliated Rui, Lower-Rank Five of the Twelve Kizuki. Defeating any member of the Twelve Kizuki is regarded as a highly impressive and difficult accomplishment, and Giyu ended Rui without even breaking stride. It's remarkable enough that Giyu invented his own form of Water-Breathing, Dead Calm, but even more impressive is the technique's obscene speed and power.
Considering that Giyu beheaded Lower-Rank Five without even the slightest hint of physical exertion, it stands to reason he could have had a better shot at defeating Akaza if he were going all-out. Displaying casual bursts of speed that make him look like he has vanished into thin air, beneath Giyu's stoic demeanor is a monster of a Hashira. While there may yet be Hashira more powerful than Giyu, for now he is pretty clearly the strongest Hashira seen thus far in Demon Slayer.