If we consider the powers and abilities in both universes in a fanfiction context, I think it could be Eri from My Hero Academia. Her quirk has the potential to be extremely powerful as it can rewind a person's body to a previous state. If she could learn to control it fully, she could potentially 'undo' the powerful undead in the Overlord universe or even reverse the magic spells. However, this is a very speculative idea as she is still a young and inexperienced character, and the power of Ainz Ooal Gown and his allies in the Overlord world should not be underestimated.
Sora would be a very powerful contender. He has a lot of experience fighting all kinds of enemies in different worlds in Kingdom Hearts. His Keyblade skills and ability to use magic would give him an edge. Also, he has the power of the heart on his side. But Izuku Midoriya with One For All at its full potential could also be extremely powerful. He has incredible strength and speed, and his determination is unwavering.
Akeno Himejima from Highschool DxD has her powerful lightning abilities. If she were to face off against someone like Bakugo from My Hero Academia, it would be an interesting match. Bakugo has his explosive quirk and his aggressive fighting style. Akeno, on the other hand, has her elegance and control over lightning. It would depend on the situation and how their powers interact in the fanfiction scenario.
It's hard to say for sure, but Naruto himself could be a very powerful contender. His vast array of jutsu and his ability to harness the Nine - Tails' power would make him extremely formidable. He could easily take on many opponents at Marineford with his shadow clones and powerful attacks like the Tailed Beast Bomb.
It could be Danny. His ghost powers give him a wide range of abilities like flight, intangibility, and ectoblasts. In the Digital World, these powers would be very useful. He could also phase through digital barriers and surprise his enemies. Also, his ability to overshadow people or Digimon could give him an edge in battles.
It's likely the Doom Slayer. He has proven to be an unstoppable force against hordes of demons. His strength, speed, and arsenal of weapons make him a formidable opponent. In a collision with Warhammer 40K, he could take on many of the threats there as well.
The Catalyst from Mass Effect might also be a very powerful entity in such a crossover. It has control over the Reapers and a vast understanding of synthetic and organic life. In a collision with the Terminator universe, it could potentially manipulate or even control the Terminators, as they are machines. It could use them to further its own goals, whatever those might be in this new combined scenario.
In a fanfiction setting, Bakugo might be the most powerful. His explosive quirk is already very destructive. At Hogwarts, he could learn some offensive magic spells and combine them with his quirk for some really powerful attacks. He's also highly competitive and would strive to be the best, which would drive him to master both his quirk and magic faster than others.
It's hard to say, but Sasuke could be a really powerful character in such a crossover. His Sharingan abilities combined with possible bending skills could make him a formidable force. He could use the Sharingan to predict an opponent's bending moves and counter them effectively.
Another strong contender could be Raiden. As a thunder god in Mortal Kombat, he has control over lightning and has great combat skills. In the Harry Potter universe, his lightning attacks could be very difficult to defend against. He could also use his powers to disrupt magic, as electricity can interfere with magical devices. His immortality also gives him an advantage, as he can keep coming back in a fight no matter how much damage he takes.
Magneto might also be extremely powerful in such a crossover. His ability to control metal could be a huge advantage in the wizarding world. He could manipulate the metal in wands, cauldrons, and other magical objects. However, he would have to deal with the unique magic that doesn't rely on physical elements all the time.