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.
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.
It could be Naruto himself. His ability to tap into the Nine - Tails' chakra gives him an incredible power boost. In the merged universe, he might be able to use the Force as well, which would make him an unstoppable force. With his speed, strength, and now potentially the power of the Force, he could easily defeat most opponents.
One way to merge the two universes in fanfiction could be through a plot device like a dimensional rift. For example, a Sith Lord could accidentally open a rift that leads to Gotham City. This would bring the dark and brooding atmosphere of The Dark Knight into the Star Wars universe. Batman might find himself in a world of lightsabers and the Force, and his detective skills could be put to the test in a whole new way.
One common theme is the clash of different power systems. The Titans' power in Attack on Titan and the Force in Star Wars are very distinct, so exploring how they interact is popular. For example, a character from Attack on Titan might discover they have an affinity for the Force.
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.
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.
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.
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.
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.