Well, it might not necessarily be Naruto. Some of the more powerful Sekirei, if they were able to adapt Naruto's immortality concept, could become extremely powerful. For example, a Sekirei with the ability to control elements like Naruto and combined with immortality could be a force to be reckoned with. However, Naruto has the advantage of his ninja training and combat experience from a very young age.
It depends on how you define 'powerful'. In terms of pure physical strength and combat ability, Gohan would be at the top. However, if we consider the Sekirei's special abilities like their unique powers related to their numbers and the ability to form bonds with Ashikabi, they have their own advantages. But overall, Gohan's Super Saiyan powers and his training with the Z - Fighters would make him the most powerful in a direct confrontation.
Ichigo from 'Bleach' could be a very powerful character in the crossover. He already has strong Shinigami powers, Hollow powers, and Quincy powers. If he were to gain Sekirei - like abilities on top of that, he would be extremely formidable. His ability to adapt and grow quickly would make him a force to be reckoned with.
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.
An Immortal Naruto in a Sekirei crossover would be quite a game - changer. Naruto has a strong sense of justice which would align well with some of the Sekirei's goals. His immortality means he has all the time in the world to master both ninja and Sekirei - related powers. He could potentially disrupt the existing system of the Sekirei Plan. His shadow clone jutsu could be used for various purposes, like scouting or distracting enemies in the Sekirei world.
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.
It's likely Naruto himself. His ability to create countless shadow clones gives him a huge advantage in battle. He can overwhelm opponents with sheer numbers. Also, his rasengan is a very powerful attack. In the Castlevania world filled with vampires and demons, he could quickly adapt his skills to take on these new foes.