It's really hard to say. Naruto has some extremely powerful characters like Naruto himself with his Nine - Tails chakra and Sage Mode. But in Yu Yu Hakusho, characters like Yusuke at his Mazoku form are also incredibly strong. It might depend on how the crossover is written. If the power - up systems are combined in a certain way, it could be a character who manages to blend both types of powers most effectively.
Well, Kurama from Naruto is an immensely powerful entity. If it could somehow interact with the spirit energy in the Yu Yu Hakusho universe, it could be a force to be reckoned with. On the other hand, Genkai from Yu Yu Hakusho has vast knowledge and powerful techniques. In a combined fanfiction universe, a character who has Genkai's knowledge and Kurama's power could potentially be the most powerful. But again, it all depends on the creativity of the fanfiction writer in how they blend the two universes' elements.
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.
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.
It's really hard to say. Goku in his Super Saiyan Blue form has incredible power and can destroy planets. But Naruto in his Six Paths Sage Mode also has amazing abilities. However, if we consider raw power, Goku might be more powerful as he can reach levels of power that can destroy entire solar systems. But Naruto's intelligence and his ability to strategize could give him an edge in a fight.
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.
It could be Naruto himself. With his vast amount of chakra and powerful ninjutsu, if he were to also gain access to the cards in Yu - Gi - Oh! Arc V, he could become extremely powerful. He could use his shadow clone jutsu to handle multiple card battles at once, and his Rasengan could be combined with the power of a strong monster card for a devastating attack.
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.
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.
Ichigo and Yusuke would be a really interesting pairing. They both have that hot - headed, determined nature. Ichigo with his Hollow powers and Yusuke with his demon powers could create some epic fight scenes together. They could also learn from each other's experiences in the spirit world and how they handle their powers.
Perhaps Daenerys Targaryen. If she were to somehow acquire vampire - like abilities, she would be an even more formidable force. Her dragons combined with vampire powers such as enhanced senses and immortality (in a sense) would make her a very powerful and dangerous character. She could use her newfound powers to rule Westeros more effectively or to take on new enemies.