Rock Lee would be a strong contender. He's dedicated his whole life to taijutsu. His ability to open the gates gives him superhuman strength and speed, which would make him a formidable fist fighter.
It's really hard to say. Ryu has his powerful Hadoken and other special moves, but Naruto has the Nine - Tails' chakra at his disposal. If we consider raw power, Naruto with his full Nine - Tails form would be extremely powerful. He could cause massive destruction with his Tailed Beast Bombs. However, characters like Akuma also have insane power levels with his Raging Demon move. So it really depends on how you define power in this crossover context.
Akuma is extremely powerful in the Street Fighter universe with his dark powers. However, in an 'rwby street fighter fanfiction', Pyrrha Nikos might be able to give him a run for his money. Pyrrha is a skilled fighter with her polarity semblance. She could potentially manipulate Akuma's attacks and turn them back on him. Also, her combat skills are top - notch, which would make her a very powerful character in this crossover.
It could be Naruto himself. His ability to use massive amounts of chakra, create shadow clones, and use powerful jutsu like the Rasengan and the Sage Mode would make him a formidable opponent. He could adapt his ninja skills to the Street Fighter fighting style easily.
Arceus might be the most powerful. As the god - like Pokemon, it has control over all types of elements in the Pokemon world. In a Street Fighter - style fanfic, it could use these elemental powers in combat. For example, it could summon lightning like Blanka, or create fireballs like Ken. Its sheer power and versatility would make it very difficult for any other character, whether from the Pokemon or Street Fighter side, to defeat.
It could be Akuma. His Satsui no Hado gives him incredible power and his fighting skills are top - notch. In a crossover, he could easily adapt to the Mortal Kombat's more brutal fighting style and use his dark powers to overpower many opponents.
Naruto himself is often very popular in 'Naruto Fist' fanfiction. Since it's related to a technique named after him, writers like to explore new aspects of his character in relation to this. He might be shown teaching others the Naruto Fist or coming up with new variations of it.
It's really hard to say for sure. Shao Kahn from Mortal Kombat is extremely powerful with his brute strength and magical abilities. But then you have characters like M. Bison from Street Fighter who has powerful psychic powers. If we consider a crossover, it might depend on the rules of the world they are in. In a world more based on Mortal Kombat's rules, Shao Kahn might have the edge, but in a more Street Fighter - like setting, M. Bison could be more dominant.
One idea could be a tournament where Naruto characters use their fist - fighting skills. For example, Naruto could face off against Sasuke in a no - jutsu, just pure fist - fighting battle. It would be intense as they both have their own unique fighting styles. Naruto's more energetic and improvisational style against Sasuke's calculated and precise strikes.
Chun - Li and Sakura could be a great pairing. They are both strong female characters. Chun - Li with her lightning - fast kicks and Sakura with her powerful punches (after she trains with Tsunade). They could exchange fighting techniques and also bond over their determination to protect their respective worlds and the people they love.
Omnimon could also be a very powerful partner for Naruto. Omnimon is a super - powerful Digimon resulting from the fusion of WarGreymon and MetalGarurumon. Naruto's ability to make friends and work in teams would be perfect for handling Omnimon's complex nature. Together, they could take on any enemy in the fanfiction world.