The following contains major spoilers for the Demon Slayer: Kimetsu no Yaiba manga by Koyoharu Gotouge, available in English through Viz Media.
The Hashira in Demon Slayer: Kimetsu no Yaiba are the most highly gifted swordsmen the Demon Slayer Corps has to offer. These nine warriors can use their superhuman abilities to go toe to toe with even the strongest of Muzan Kibutsuji's Upper-Rank demons. All Hashira are equals in title, if not necessarily in strength.
Some, like Gyomei Himejima and Muichiro Tokito, are so strong that even other Hashira look at them with awe. However, a handful of these swordsmen are actually somewhat weak, at least by Hashira standards. Here's a look at three Hashira who are obscenely powerful Demon Slayers, yet who are still the weakest swordsmen to hold the prestigious title.
Mitsuri Kanroji Is a Great Meat Shield
The protégé of the late Kyojuro Rengoku, Mitsuri Kanroji is poised to enter the spotlight in the anime's upcoming "Swordsmith Village" arc. The Love Hashira, Kanroji developed her signature Love-Breathing during her time as Rengoku's Tsuguko (apprentice) when she realized that the more traditional Flame-Breathing did not suit her. Her muscle density is eight times higher than the average human, meaning her slender frame still possesses more strength than even the most muscular warrior imaginable. While Kanroji is a great swordswoman with her unique, whip-like katana, it's her toughness that makes her Hashira-worthy, even if she failed to accomplish much throughout the series.
During the "Swordsmith Village" arc, Kanroji arrived just in time as Tanjiro Kamado, his sister Nezuko, and Genya Shinazugawa were being overwhelmed by Hantengu, Upper-Rank Four of the Twelve Kizuki. Kanroji fought off his most powerful form singlehandedly, with even Hantengu expressing astonishment that she'd survived his most powerful attack at point-blank range.
Fighting Upper-Rank Four without dying or being permanently disfigured is no small feat, but Kanroji didn't do much other than stall for time, as her durable body still took a savage beating while Tanjiro delivered the final blow elsewhere on Hantengu's main body. During the manga's final arc, "Sunrise Countdown," Kanroji was one of the last Hashira standing to face Muzan but was mortally wounded almost immediately, ultimately contributing very little to the final battle against Muzan and the Upper-Ranks.
Tengen Uzui's Self-Assessment Is Harsh but Accurate
During the "Entertainment District" arc, the Sound Hashira laughed off the idea that he was naturally gifted, proclaiming that Gyutaro (Upper Six) needed to get out more if he thought Tengen was special. While Tengen Uzui certainly did have self-esteem issues, his battle against Upper-Rank Six supported his claim that he's nothing compared to some of the other Hashira.
Iguro Obanai, the Serpent Hashira, backhandedly complimented Tengen for defeating the "lowly" Upper-Six, indicating that a stronger Hashira would have fared much better against Gyutaro and Daki. Mitsuri Kanroji and Muichiro Tokito fought Upper-Ranks Four and Five respectively and recovered to full strength in a few days, while Tengen was permanently disfigured and forced into retirement after battling Upper-Rank Six, who was so much weaker than the other Upper-Ranks that even Muzan wasn't surprised upon learning of his death.
Shinobu Kocho Is the Weakest Hashira
The Insect Hashira, Shinobu's petite frame prevents her from being physically able to decapitate any demon, be it an Upper-Rank or the weakest demon in Japan. At one point or another in the manga, all nine Hashira in Demon Slayer encountered an Upper-Rank. Of the three Hashira who were killed by Upper-Ranks -- Rengoku, Shinobu and Tokito -- Shinobu was the only one who was killed almost instantly when she encountered Doma, Upper-Rank Two. The most tragic part of Shinobu's demise was the fact that even Doma admitted that she was certainly fast enough to defeat him if she only had the strength to decapitate him.
In Shinobu's defense, her poison successfully weakened Doma to the point that Inosuke Hashibira and Kanao Tsuyuri were able to decapitate him. When Doma consumed Shinobu, she had 37 kilograms of Wisteria poison coursing through her body, and a flashback revealed that it was her intention to be eaten by him. However, the idea that a kamikaze plan was her only chance of defeating an Upper-Rank demon -- something that Gyomei, Sanemi, Muichiro, Tanjiro and even Zenitsu managed to do without killing themselves -- means that Shinobu Kocho was fairly weak by Hashira standards.