Hermione might be a strong candidate. She is extremely intelligent and has a thirst for knowledge. In a merged world, she would quickly study the new elements from the God of War universe. She could combine her magic knowledge with the new combat or magical concepts from God of War. For example, she might figure out a way to use the power of the Greek gods in the wizarding world, making her a very powerful and unpredictable force.
It could be Naruto. His ability to use chakra and his many powerful jutsu like the Rasengan and the Sage Mode would give him an edge. He also has great determination and a never - give - up attitude. He could probably adapt to the magic in the Harry Potter world quite well and use it in combination with his ninja skills.
Doctor Strange would be a very powerful contender. His mastery over the mystic arts in the Avengers universe gives him abilities like time manipulation, astral projection, and creating portals. However, in the Harry Potter universe, Dumbledore is extremely powerful with his vast knowledge of magic, his ability to use powerful spells like the Patronus charm on a large scale, and his strategic mind. So it's really hard to say who would be the most powerful as they both have different strengths.
Harry Potter himself could also be a very powerful character in the merged universe. He has his own magical skills and the experience of fighting dark wizards. He could teach the Stranger Things characters about the wizarding world and use his magic to protect them from the Upside Down threats. Also, his connection to Voldemort might give him some unique insights into defeating any dark forces that come from the merging of the two universes.
It would probably be Harry Potter himself. He has the power of magic, which is extremely versatile. He could use spells like 'Expelliarmus' to disarm enemies or 'Accio' to call weapons to his side from a distance in the midst of a Call of Duty - style battle.
It could be Harry Potter himself. He has defeated Voldemort, one of the most powerful dark wizards. His ability to perform advanced magic like the Patronus charm and his experience in facing great evils would give him an edge. He could also learn from the Olympian magic and become even more powerful.
It's likely Goku. His ability to constantly reach new levels of power like Super Saiyan God and Ultra Instinct is far beyond what most My Hero Academia characters can achieve. Even the strongest in My Hero Academia, like All Might at his prime, would have a hard time against Goku. Goku can destroy planets, while the power in My Hero Academia is more focused on individual combat and saving cities rather than galactic - level threats.
Saitama is clearly the most powerful. In the Attack on Titan world, the Titans are a huge threat. But for Saitama, they are just like any other weak opponent. He doesn't even break a sweat dealing with the most powerful threats in his own universe, so the Titans would be no match for him. His power is not only physical but also his unshakable will and his simplicity in approaching any fight. He doesn't get distracted or intimidated, which makes him an unstoppable force in any situation, including in a merged OPM and Attack on Titan universe.
Some might argue that Four from 'Divergent' could be the most powerful. His combat expertise and leadership skills could be very useful in the Hunger Games setting. He could potentially lead a group of tributes to victory, using his knowledge of strategy from both worlds.
It's hard to say for sure, but Naruto in his Sage of Six Paths mode is extremely powerful. However, Yuno from Future Diary has her own set of very deadly and unpredictable powers. If she were to target Naruto without him knowing, she could potentially be a huge threat. But Naruto also has the ability to sense danger and his large amount of chakra might give him an edge in a long - drawn - out battle.
One might argue that the Sage of Six Paths from Naruto would be the most powerful. He has the ability to create and shape the world. But in the context of a merged universe with World of Warcraft, the Lich King also has great power. He commands the undead and has powerful necromantic abilities. If they were to face off, the Sage of Six Paths' positive energy - based powers might be at odds with the Lich King's necromancy. It would be a battle of two very different but extremely powerful forces.