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.
Saber is also a strong contender. Her sword skills and noble phantasm are extremely powerful in 'Fate Stay Night'. If she were in the 'Harry Potter' world, she could easily cut through dark wizards. And her code of honor might make her a respected figure among the Hogwarts students and teachers, which could give her an advantage in terms of alliances.
Another possibility is Merlin. In the 'Harry Potter' lore, he's a very powerful wizard. In a combined universe, he could use his magic to control or even enhance the Servants. He might be able to use his knowledge of ancient magic to disrupt the 'Fate Stay Night' magic system and become the most powerful force, especially if he were to side with the forces of good.
Well, if we consider Harry Potter himself, he has the power of love and the ability to face great evil. But when compared to some of the Servants in 'Fate Stay Night', he might seem less powerful at first glance. But his determination and the fact that he has overcome death multiple times could give him an advantage. On the other hand, a character like Archer from 'Fate Stay Night' has a wide range of skills and the ability to analyze situations quickly. In a combined fanfiction, it would depend a lot on the situation and how the powers interact. For example, if it's a battle of wits, Dumbledore or Archer might have the upper hand, but if it's a battle of pure power, Gilgamesh or a magically enhanced Harry Potter could be the victor.
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 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.
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.
I think Harry Potter and Saber would make an interesting pairing. Harry has his own sense of justice and courage, and Saber is a noble and powerful Servant. They could learn from each other's leadership styles. Another good pairing could be Hermione and Rin. Both are intelligent and skilled in magic. They could exchange knowledge about their different magical worlds and maybe even come up with new spells or magical techniques together.
It's hard to say for sure, but Saber from Fate Stay Night would be a very strong contender. Her combat skills and noble phantasm are extremely powerful. However, in the context of a crossover, Subaru's Return by Death ability could potentially give him an edge. He could keep trying different strategies against her until he found a way to defeat her.
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.