It could be Naruto. His ability to use the Nine - Tails' chakra gives him incredible strength, speed, and durability. In a crossover, he could easily adapt his ninja skills to deal with the demons in Kimetsu no Yaiba. His Rasengan could be a very effective weapon against them.
Saitama from One - Punch Man (just kidding, not in the actual crossover). Seriously though, it might be Kaguya from Naruto. She has god - like powers and could easily overpower most of the characters in Kimetsu no Yaiba. Her ability to create dimensions and use powerful ninjutsu would make her a formidable force in this crossover.
A very creative 'kimetsu no yaiba crossover fanfiction' idea is the combination with 'Doctor Who'. The time - traveling aspect of 'Doctor Who' allows for some really out - there storylines. The Doctor could team up with the Demon Slayers at different points in time and space, perhaps even preventing some demon - related disasters before they start. It's a concept that mixes two very different universes in a really cool way.
Well, a cool plot might be a scenario where the Sharingan from Naruto has some sort of special interaction with the Breathing Styles in Kimetsu no Yaiba. For example, Sasuke travels to the world of Kimetsu no Yaiba. He discovers that his Sharingan can analyze and enhance the Demon Slayers' Breathing Styles. In return, the Demon Slayers teach Sasuke some unique swordsmanship techniques that he can incorporate into his own fighting style.
It's really hard to say. Naruto in his strongest form, like when he has the power of all the tailed beasts, is extremely powerful. He has incredible strength, speed, and the ability to use a vast array of jutsu. However, Aang in the Avatar state has access to all four elements and can be a formidable opponent. If we consider the Avatar state's spiritual connection, it could give Aang an edge in a battle of wills.
Sesshomaru could also be a very powerful character in this crossover. He has his own powerful sword, Bakusaiga, and his inhuman strength and speed. He is very experienced in combat against demons. Compared to Naruto's friends like Sakura, Sesshomaru would likely be much more powerful in terms of raw strength and combat ability. But Sakura has her own medical ninjutsu and superhuman strength when she releases her inner power, which could also be useful in different situations.
There are quite a few 'kimetsu no yaiba crossover fanfiction' that are worth checking out. For example, some crossovers with 'Fullmetal Alchemist'. In these stories, the alchemy concepts blend with the demon - slaying world of 'kimetsu no yaiba', creating new and exciting scenarios. Also, the crossovers with 'My Hero Academia' are cool as it shows how the Demon Slayers would fit into a world of superheroes.
Some might consider a crossover with the One Piece world. The Straw Hat Pirates and the Demon Slayers could have an adventure together on the Grand Line, facing both pirates and demons. A crossover with the Sword Art Online world is also possible. The Demon Slayers could enter the virtual world and face new kinds of threats there. Additionally, a crossover with the Fate series would bring together different heroic spirits and Demon Slayers in a battle royale - like scenario.
Sasuke would also be very powerful. His Sharingan gives him enhanced perception and the ability to copy moves. In the SAO world, he could quickly analyze the fighting styles of opponents and come up with counter - strategies. His Chidori and other lightning - based jutsu could be very effective against the digital enemies in SAO.
However, we can't forget about Steve. He has been living in the Minecraft world for a long time. He has in - depth knowledge of how to build the most powerful weapons and structures. He can use his pickaxe with great skill, which could be as effective as a ninjutsu in some cases. Also, his ability to survive in the Minecraft world's harshest conditions gives him an edge over the Naruto characters who are new to this environment.
It could be Naruto in his Sage of Six Paths mode. He has incredible power, with the ability to fly, use various powerful jutsus, and his chakra is off the charts. In a fight against the characters from Toriko, his speed and strength would be very difficult to match.
In a naruto tekken crossover fanfiction, I think Orochimaru could be a very powerful character. He has the ability to use various snake - based jutsu and is a master of deception. He could potentially use his skills to outwit Tekken characters. But on the Tekken side, characters like Heihachi Mishima with his Mishima - style karate and his decades of fighting experience would also be a formidable force. It really depends on the environment and the nature of the encounter.