Doma is considered as the third strongest demon in the series. The combination of his battle techniques and skills is powerful enough that he can take out a Pillar in Tanjiro's era without any difficulty. In the past, he beat Kanae Kocho, the previous Flower Pillar, and the older sister of Shinobu Kocho.
Yoriichi Tsugikuni deserves the honor of being the strongest Demon Slayer to ever exist. Unlike other great warriors, who may or may not acquire their Demon Slayer Marks later in life, Yoriichi was born with it. Furthermore, this character singlehandedly invented the original Sun Breathing Style.
1. Gyomei Himejima. 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.
Shinobu Kocho Is the Weakest Hashira
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.
In conclusion, Tanjiro will not be able to kill Muzan by himself since the Demon King is far superior in every aspect of combat. The manga has shown fans the singular way that Muzan could be bested and it was only due to the collective efforts of Demon Slayer Corps members.
Tanjiro decapitates Akaza with the Hinokami Kagura: Setting Sun Transformation. Akaza was bewildered, wondering how Tanjiro did not have any fighting spirit, comparing this feeling to fighting against a plant. He acknowledges that Tanjiro surpassed his speed.
Before he is able to regenerate fully, Tamayo appears and uses her Blood Demon Art to keep him in place, while using a drug that can turn him into human. Using this opportunity, Gyomei Himejima appears and destroys Muzan's head.
While she might not be as powerful as Muzan himself, her overall strength makes her easily comparable to an Upper rank demon.
Muzan Kibutsuji has done some terrible things in Demon Slayer, but his slaughter of Tanjiro's family is one of the most mysterious.
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.
1 Kyojuro Rengoku
Despite his limited time in Demon Slayer, the Flame Hashira Kyojuro Rengoku established himself as the series' best character, and fans dearly miss him. He was the star of the Mugen Train story arc, though he also appeared in a double-length episode before that.
Tanjuro Kamado a recurring flashback hero from Demon Slayer : Kimetsu No Yaiba. He was the husband of Kie Kamado, and the father of Tanjiro, Nezuko, Takeo, Hanako, Shigeru, and Rokuta.
The only thing he's weak to is the sun. That is why he turned other humans into demons and spread his blood so that one of them might find a solution against the sunlight, so he can become truly perfect.
Zenitsu is clearly on the same level as Hashira. If we compare him to Shinobu, his speed is far better, while she lacks quite a lot in raw power. There is a good chance that he could beat Shinobu if the two decide to fight.
Tanjirou and Giyuu are the first one to approach him. There they fight Muzan Together without stopping. They realized that the only way to kill Muzan is to drag him into the sun.
Both Obanai and Mitsuri give their all in this battle with their Flower Breathing and Serpent Breathing techniques to heavily contribute in killing the evil existence that was Muzan.
Since he didn't know anything about the Blue Spider Lily medicine, he needed others to find the plant for him by happenstance. This is the general reason why Muzan turned Nezuko into a demon. More specifically though, was the fact that Nezuko was of Kamado blood.
Tanjiro Is Strong, and Stronger than Zenitsu as he uses Sun Breathing.
Obanai Iguro is one of the harsher Hashira members. He demanded that Giyuu be punished for allowing Nezuko to get away unharmed, and later restrained Tanjiro as he tried to help his sister. He hates demons and has little regard for his fellow Hashira, except Mitsuri Kanroji later on.
Shinobu Kocho (The Insect Hashira) While we don't necessarily think Shinobu Kocho is physically weak by any means, we do believe that she is currently positioned at the bottom of the totem pole compared to her fellow Hashira members.