The concept of Breathing Styles makes the characters in Demon Slayer so much more powerful than the average person. They each have a unique fighting style that fits their preferences, making them capable of taking on any challenge.
By combining these Breathing Styles and the indomitable will many Demon Slayer characters have, there is a multitude of incredibly strong sword users in the series.
Note: This list is unranked and contains massive spoilers for the Demon Slayer manga. It is also subjective and reflects the author's opinion.
10 of the strongest swordsmen in Demon Slayer include Sanemi, Kyojurou, and others
1) Demon King Tanjiro
Demon King Tanjiro is the strongest swordsman and character in the series. Although his time was short-lived, he quickly made his mark on the Demon Slayer world. At this point, Tanjiro was essentially Muzan with immunity to the sun. It took a miracle for him to be defeated, but if the Demon Slayers had lost, the world might have ended.
2) Yoriichi Tsugikuni (Progenitor of Breathing Styles)
Yoriichi Tsugikuni has proven himself to be one of the strongest swordsmen and characters in the entire series. In fact, he was the strongest before Demon King Tanjiro was created. He was capable of cleaving up Muzan using Sun Breathing, almost killing him without any difficulty. If he had not hesitated to end his life, the Demon Slayer series would not exist.
3) Kokushibo (Upper Moon One)
Kokushibo, once known as Michikatsu Tsugikuni, is the older brother of Yoriichi. Although he was never able to reach his younger brother's level of strength, Kokushibo was second only to Muzan during the present-day events of the Demon Slayer series. He utilized Moon Breathing and was capable of outclassing each of the Hashira in combat.
4) Sanemi Shinazugawa (Wind Hashira)
Sanemi is the strongest swordsman amongst all of the Hashira. His rare marechi blood allows him to impair their cognitive functions, essentially making them drunk. Sanemi is able to easily shred through demon after demon with his Wind Breathing and aggressive fighting style.
5) Obanai Iguro (Snake Hashira)
Despite his lack of physical strength, Obanai is able to take on powerful demons with his incredible skill. His mastery in Snake Breathing and twisted nichirin katana allow for him to make unexpected attacks on demons. He was even able to keep up with Muzan and cut off one of his arms during the final fight, despite having his eyes destroyed.
6) Kyojurou Rengoku (Flame Hashira)
Rengoku was a highly enthusiastic and loud individual. As the Flame Hashira, he underwent grueling training in order to become incredibly powerful. Each of his attacks packs a wild punch and is capable of pushing any demon back. Sadly, he died fighting Akaza after the defeat of Lower Moon One, Enmu, and this had fans fall into a frenzy.
7) Tomioka Giyuu (Water Hashira)
Tomioka Giyuu is a skilled swordsman, having trained under Sakonji Urokodaki himself. He has mastered every Water Breathing form and even created his own, known as Eleventh Form: Dead Calm. During the final battle, he was able to keep up with Muzan along with his fellow Hashira and eventually defeat him.
8) Mitsuri Kanroji (Love Hashira)
Mitsuri is not the best in terms of skill, but she has amazing physical strength. She was able to pull off one of Muzan's arms with her bare hands during the final battle. Her unmatched flexibility and whip-like nichirin katana allow her to take down enemies from any range.
9) Muichiro Tokito (Mist Hashira)
Muichiro is a descendant of Kokushibo and a truly talented individual. He was able to reach the Hashira position at the age of 14 and killed Upper Moon Five: Gyokko without any difficulty. Like Giyuu, Muichiro became so skilled in his Breathing Style that he was able to create a new technique known as Seventh Form: Obscuring Clouds.
10) Tengen Uzui (Sound Hashira)
Tengen Uzui is the flashiest character in the series. His experience as a Shinobi and Hashira made him a formidable opponent against the Upper Moon Six duo, Daki and Gyutaro. His master swordsmanship and poison resistance allowed him to overpower the two in the end and win the fight.