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.
Thor could also be very powerful. He has his Asgardian strength and his hammer (or Stormbreaker). If he were to also gain the ability to use magic like the wizards in Harry Potter, he could be near - invincible. He could call lightning while also casting spells. And his knowledge of different realms could give him an edge in any battles that span across the combined universes.
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.
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 could be Albus Dumbledore. He has great magical knowledge and power in the 'Harry Potter' universe. When combined with the 'Fate Stay Night' elements, he might be able to outwit even the most powerful Servants. His ability to plan and his understanding of magic could give him an edge in any conflict.
It's hard to say for sure, but Ruby could be a very powerful character. Her speed and scythe skills are already formidable in the RWBY universe, and if she were to gain some of the combat knowledge from For Honor, like the strategic thinking of a For Honor general, she could be extremely powerful. She might be able to quickly analyze her opponents' fighting styles and counter them effectively.
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. He has an enormous amount of Chakra and his ability to learn new techniques quickly. If he were to partner with a powerful Digimon like Omnimon, he would be nearly unstoppable. His ninja skills combined with Omnimon's power would make him a force to be reckoned with.
Gohan in his Ultimate form could also be a very powerful character in this combined fanfiction universe. He has incredible strength and intelligence in combat. But on the RWBY side, Ozpin has a lot of hidden power and knowledge. Still, when comparing their power levels, Gohan's physical strength and energy projection abilities are on a different scale. He could potentially take on even the toughest Grimm with ease. So, overall, Gohan might be among the most powerful in this crossover scenario.
It's hard to say for sure, but Master Chief could be a strong contender. His combat training and advanced armor give him an edge. However, Darth Sidious is extremely powerful with his dark side abilities. He could potentially manipulate Master Chief if he got the chance. But then again, the Forerunners from Halo have such advanced technology that they might be able to counter the Force -powers. So it really depends on how the story is written.
I think it could be a combined form of a character. For example, if Issei were to somehow gain the power of the Nine - Tailed Fox. He already has his Boosted Gear and his experience in battles in the Highschool DxD world. Adding the Nine - Tailed Fox's chakra would make him an extremely formidable opponent. He could use the fox's chakra to enhance his Sacred Gear even further, and his new form might have abilities that are a combination of both universes, like using the fox's tails as a weapon while also having the power of the Boosted Gear.