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.
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.
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.
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.
However, some might argue that Levi Ackerman could hold his own. His incredible speed and combat proficiency with blades could potentially make him a match for some of the Star Wars characters. If he were to get his hands on a lightsaber, he might become an even greater threat. But in the grand scheme of things, when facing the more powerful Force - using characters, he might still be at a disadvantage.
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's hard to say for sure, but some might argue that it could be Julius Novachrono from Black Clover. His time - magic is extremely powerful, and he could potentially manipulate time to his advantage in battles against the Highschool DxD characters. He could stop time and land fatal blows before his opponents even realize what's happening.
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's hard to say for sure, but Dante would be a strong contender. His experience in fighting demons and his vast array of weapons and abilities make him a formidable force. He has his devil trigger which significantly boosts his power, and his swordsmanship is top - notch. However, Naruto also has his own strengths. With the power of the Nine - Tails and his ability to use various powerful jutsu, he could also hold his own. But if we consider adaptability to different types of enemies, Dante might have an edge as he's used to facing all kinds of demons.