Demon Slayer has introduced a wide variety of combat-ready characters on both sides of the endless feud between humanity and Muzan Kibutsuji's demon hordes. The best Demon Slayer characters all learn and grow as they fight throughout this anime, such as developing new combat techniques and shoring up their greatest weaknesses to be better balanced.
By the end of Season 3, the Swordsmith Village arc, Demon Slayer's best characters have done some impressive work to better themselves as fighters. Tanjiro Kamado can now take on the Upper Moons and win, but even so, he has a ways to go. Tanjiro and his fellow demon slayers still have room for improvement as they address their biggest weaknesses, and that may help define their arcs in future seasons/movies in Demon Slayer.
10 Tanjiro Kamado
Protagonist Tanjiro Kamado used Water Breathing in most of Season 1, a fighting style with fluid, sweeping motions that suited him well. Then Tanjiro started using Hinokami Kagura, a powerful fighting style with flame-like movements, strong enough to take on the likes of Daki and Gyutaro. However, Hinokami Kagura puts a heavy burden on Tanjiro's body.
Slowly but surely, Tanjiro is getting better at using Hinokami Kagura, though he still has a ways to go. Ideally, Tanjiro will train further with that devastating technique so he can use it repeatedly in fights against the Upper Moons without exhausting himself.
9 Nezuko Kamado
Nezuko Kamado never meant to become a demon, but she has gotten used to it and has figured out how to assert her strength as a creature of the night. In combat, Nezuko shines in melee combat when she uses her claws and powerful kicks, and she can power up in her adult form, as her Entertainment District duel against Daki showed.
Nezuko has all the typical strengths of demons, and she even overcame her weakness to sunlight, but she still has other deficiencies to consider. She never received formal combat training, meaning her technique is aggressive but sloppy, so she could afford to train with martial arts masters before fighting again.
8 Zenitsu Agatsuma
Tanjiro's dandere friend Zenitsu Agatsuma has been openly afraid of combat since he was first introduced, and he refused to even try fighting. He didn't show his strength until he passed out from fear in Kyogai's house, allowing Zenitsu to perform Thunder Breathing while asleep.
Zenitsu will get stronger if he can use the full extent of Thunder Breathing at will rather than needing to sleep, and he may unlock stronger forms of Thunder Breathing in the process. He did well against Daki with Gyutaro's aid, but there's still more ground to cover, especially if Zenitsu learns to overcome his intense fear of demons.
7 Inosuke Hashibira
Tanjiro's aggressive friend Inosuke Hashibira was already a highly capable demon slayer when they met right after Kyogai's defeat in Season 1. Inosuke boasted ultra-sharp instincts, his own breathing style called Beast Breathing, and a tough, mobile body that could keep up with any demon. He also has two Nichirin swords rather than one.
Inosuke has already mastered his own fighting style and has few distinct weaknesses, so his next step is to awaken a demon slayer mark. Given how Inosuke likes to live on the edge and is attuned to his instincts, pressuring himself into awakening that mark the way Mitsuri and Muichiro did should be easy.
6 Genya Shinazugawa
Genya Shinazugawa is an unusual demon slayer who couldn't learn a breathing style of any kind, so instead, he learned the Repetitive Action technique from Gyomei Himejima, the Stone Hashira, to boost his power. Genya also uses a double-barreled shotgun loaded with anti-demon ammo, though it wasn't enough to put down Hantengu, the Upper Moon 4.
Genya has ample room for improvement in various arenas. He could upgrade his firepower with a second shotgun to obliterate demons up close and/or get stronger ammo to seriously harm an Upper Moon. He could also stand to awaken his own demon slayer mark or, like Inosuke, at least try to invent his own breathing style if he can't learn the conventional ones.
5 Muichiro Tokito
When he first appeared in the Swordsmith Village arc, the Mist Hashira Muichiro Tokito was an apathetic and oddly airheaded boy who took no interest in others. He just focused on his own training, but when Gyokko and Hantengu made their move, Tanjiro inspired Muichiro to fight for others and not just for duty's sake.
Muichiro was inspired to think back on his early days with his brother, and that helped him awaken his demon slayer mark. Now, Muichiro might get stronger again if he can choose someone to personally fight for, drawing upon the shonen-style power of friendship or even the power of love.
4 Shinobu Kocho
Shinobu Kocho, the quietly intense Insect Hashira, is known for her use of deadly wisteria-based poisons to defeat demons since she lacks the strength to cut off their heads with her Nichirin sword. She is also highly agile and is adept at using thrusting moves like a fencer to "sting" her enemies.
Shinobu could awaken her demon slayer mark to boost her power, but that's not her only option. Before the final fight against Muzan, Shinobu could undergo strength training to become powerful enough to chop off demon limbs as an extra option in battle. She could also develop ever-stronger poisons to take on the Upper Moons in case her current concoctions are becoming obsolete.
3 Kanao Tsuyuri
Kanao Tsuyuri is Shinobu Kocho's dandere apprentice at the Butterfly Estate. She is a quiet but highly capable demon slayer who uses her remarkable agility and technique to outmaneuver her opponents, though her offensive power hasn't yet been shown in the anime. Fans do know that she uses the Flower Breathing style, however.
Kanao might get stronger if she simply becomes more assertive and bold in battle, which would allow Kanao to use the full extent of her formidable skills even in the face of an Upper Moon. She might also awaken her demon slayer mark and, if necessary, learn more Flower Breathing moves or invent new ones before the final battle begins.
2 Mitsuri Kanroji
Mitsuri Kanroji is the Love Hashira and was first seen in action in the Swordsmith Village story arc. She has incredible muscle density and relies on her sheer strength and endurance to win, all while using her whip-like Nichirin sword to fight at a medium distance. She also awakened her demon slayer mark around the same time Muichiro awakened his.
Mitsuri only has minimal room to grow, but she'll need it for the final fight. She could practice Love Breathing's final forms, such as forms 9 and 10, to unleash her full power against foes even stronger than Hantengu. She must also make sure she can use her demon slayer mark at will.
1 Aoi Kanzaki
Aoi Kanzaki is an associate of the Butterfly Estate, where she helps Shinobu Kocho train anyone who needs to sharpen their technique. Aoi actually underwent training as a demon slayer, and she even took the final exam, but she hasn't done much as a slayer since then aside from training others.
If necessary, Aoi can and possibly should resume her own training before the final fight with Muzan to make sure the demon slayer corps can maximize her firepower. Aoi may be rusty at Water Breathing, but it's not too late for her to train with Tanjiro and the others to take on Muzan's remaining Upper Moons. If she can awaken a demon slayer mark, all the better.