Madara Uchiha could be a top contender. With his Sharingan and Rinnegan, he has powerful visual abilities that could give him an edge in the SAO world. He could use his Susanoo as a powerful weapon in virtual battles. And his strategic mind and vast combat experience from the Naruto universe would make him a very difficult opponent to defeat, even when facing the skilled players of SAO.
Kirito has a strong case too. He has a lot of experience in virtual reality combat and is a very skilled swordsman. In a merged universe, he could potentially learn some ninja techniques from Naruto and combine them with his own sword skills. He is also very adaptable and intelligent, which would help him face off against powerful Naruto characters. However, Naruto's raw power might still overshadow him in the end.
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.
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.
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.
It's hard to say for sure, but Naruto himself could be a strong contender. His vast array of jutsu and his ability to grow in power constantly could make him extremely powerful. He has faced incredibly tough opponents and come out on top in the Naruto universe, so in a merged universe, he could use his skills to take on the devils and other beings.
Kirito has a good chance of being very powerful in this crossover. He has a lot of experience in Sword Art Online, and his dual - wielding skills are top - notch. In a fight against Naruto characters, he could use his speed and precision to dodge attacks and then counter - attack. However, he would have to deal with the unpredictable nature of ninja abilities like genjutsu, which could be a challenge for him.
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.
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.