Well, it could be Sirzechs Lucifer from 'high school dxd'. He is a very powerful devil with great leadership skills and combat abilities. But then again, Kratos has a track record of taking down some of the most powerful deities. If we consider Kratos' rage and his ability to adapt in combat, he could potentially overpower Sirzechs. But in the context of the 'high school dxd' world's magic system, Sirzechs might have an edge in terms of strategic use of his powers.
It's hard to say for sure. Kratos is extremely powerful in the 'god of war' series. He has slain gods and Titans. However, in 'high school dxd', Great Red is an incredibly powerful being that exists on a different level. If they were to face off, Kratos' combat experience and physical strength might be pitted against Great Red's otherworldly and near - omnipotent powers. It could go either way depending on how the fanfiction writer decides to balance their abilities.
Another contender could be Ophis from 'high school dxd'. She is an extremely ancient and powerful being. Her power level is off the charts. But Kratos has faced and defeated beings with god - like powers. In a fanfiction scenario, if they were to meet, Ophis' dragon - like powers and her control over the power of infinity would be a huge challenge for Kratos. However, Kratos' indomitable will and his arsenal of weapons could also pose a great threat to Ophis.
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.
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.
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.
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.
If we consider the character of Harry Potter himself, his ability to resist the Killing Curse and his connection to the Deathly Hallows could make him a formidable force. In the world of Bioshock Infinite, he could use his magic to protect the oppressed and fight against the tyrannical rule in Columbia. His courage and the power of love, which are important themes in his own story, could also play a significant role in this crossover scenario, perhaps even influencing the characters in Bioshock Infinite in unexpected ways.