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.
Dumbledore could also be a strong contender. His vast knowledge of magic and his ability to think several steps ahead would make him a powerful figure in the merged universe. He could use spells like 'Fidelius Charm' to protect important locations or 'Patronus' to fend off dark forces. And his wisdom could guide the Call of Duty soldiers in dealing with the magical threats.
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.
Perhaps a combination of Soap MacTavish from Call of Duty and Harry Potter himself. Soap is a highly skilled and experienced soldier, able to handle all kinds of combat situations. Harry has his unique magical abilities and his experience in fighting dark forces. Together, they could use their respective skills. Harry could use magic to deal with large groups of zombies at once, while Soap could use his combat skills to take out any remaining threats or handle any non - magical obstacles in their path.
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.
Probably Great Red from Highschool DxD. He's already an extremely powerful being in his own universe. If he were to encounter the power of the Omnitrix and all the alien forms in Ben 10, he could potentially assimilate or use those powers in some way to become even more powerful.
It could be Issei if he were to gain Naruto's abilities. He already has his own set of powerful skills in the DxD world. Adding Naruto's chakra and jutsus like the Rasengan or the ability to summon toads would make him extremely formidable.