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.
Akuma from Street Fighter is also a very strong candidate. His powerful dark hadou attacks and his almost unrivaled martial arts skills are extremely dangerous. If he were to face Naruto characters, he might be able to use his skills to counter their chakra - based attacks. However, he would also have to deal with the unpredictability of Naruto's clones and jutsu.
Sasuke Uchiha would also be a top contender. With his Sharingan, he can predict his opponents' moves, and his Chidori is a very powerful attack. In a combined Naruto - Street Fighter world, he could learn new fighting techniques from Street Fighter characters and enhance his already dangerous skill set. He might be able to use his Sharingan to analyze the fighting styles of Street Fighter characters and find their weaknesses quickly.
It could be Akuma. He is extremely powerful in the Street Fighter universe with his dark Hadou powers. In a combined universe, his ability to manipulate dark energy could be enhanced by the magic in the Harry Potter world. He could potentially learn spells and use them in combination with his fighting skills, making him an almost unstoppable force.
Probably a combination of Deku and Ken. Deku's One For All power gives him incredible strength and speed, and Ken's Shoryuken and other moves would add a whole new level of combat ability. Deku could use Ken's fighting style to better control his power, and Ken could benefit from Deku's raw power. They could take on any enemy in the combined universes with ease.
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.
Perhaps Naruto combined with Toph. Naruto's agility and combat skills along with Toph's earthbending mastery would be quite a combination. Toph could sense enemies from afar using her earthbending senses and Naruto could quickly engage them in combat. Also, Naruto could use his clones to help Toph with larger - scale earthbending projects, like building massive structures for defense or offense in a battle.
However, one could also argue that Alien X from Ben 10 Omniverse, if combined with Naruto's willpower and determination, would be the most powerful. Alien X has reality - warping abilities, and Naruto's strong spirit could potentially control and direct those powers in ways that we can't even imagine in the context of the fanfiction.
There's also the idea that a character like Luigi, who is often overlooked but has his own unique skills, could become very powerful when combined with Naruto's abilities. Imagine Luigi using the invisibility power - up along with Naruto's stealth skills. He could be a formidable opponent in this fanfiction crossover.
It's hard to say for sure, but many would argue that Naruto could be extremely powerful. His ability to use chakra, perform various ninjutsu, and access the power of the Nine - Tails gives him a huge edge. However, characters like Thor with his godly powers and the Hulk with his incredible strength also can't be underestimated.
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.