Some might argue that Zeref from Fairy Tail could be the most powerful. He has the power to manipulate life and death. But when you think about the sheer destructive power of the likes of Beerus from Dragon Ball Super, who can destroy planets with a flick of his finger, it's hard to say. Zeref's powers are more complex and insidious, while Beerus' are more straightforwardly destructive. It really depends on the context of the fanfiction and how the author wants to play with their powers.
In a merged universe, there's also the possibility of a new character emerging as the most powerful. Maybe a fusion between a Dragon Ball Super character and a Fairy Tail character. For example, a fusion between Gohan and Gray. This new character could inherit the best of both worlds - Gohan's potential and Gray's ice - making magic. They could develop new and unique powers that make them the most powerful in the combined universe.
It's a tough call. Goku from Dragon Ball Super has the ability to reach insane power levels like Ultra Instinct. But on the other hand, Acnologia from Fairy Tail is extremely powerful with his dragon - like powers. However, if we consider the potential of Saiyan power growth and Goku's ability to adapt in battle, he might have an edge over Acnologia in a long - drawn fight.
Gray Fullbuster from Fairy Tail might be a strong contender. If he were to learn some of the chakra - based techniques from Naruto, like the Rasengan, and combine it with his ice - make magic, he could create some really devastating attacks. His ability to adapt quickly in battle would also give him an edge in a merged universe.
It's likely Goku. His Super Saiyan forms give him incredible power, and his ability to constantly train and get stronger is unrivaled. He could easily defeat most of the Kung Fu Panda characters in a one - on - one fight.
In a combined fanfiction universe, it could be Whis from Dragon Ball Super. He is an angel who is extremely powerful and has knowledge far beyond most characters. He could potentially use his knowledge of the multiverse and his own powers to outwit and overpower anyone. Although Sailor Moon has the power of the Silver Crystal and Goku has his Saiyan strength, Whis' ability to manipulate time and space, as well as his overall power level, might make him the most powerful character. However, it also depends on how the author of the fanfiction wants to balance the powers of the two universes.
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.
It's hard to say, but one could argue that a merge between Gildarts from Fairy Tail and Kakashi from Naruto would be extremely powerful. Gildarts has incredible magic power and combat skills, and Kakashi is a highly skilled ninja with the Sharingan. Their combined abilities in terms of combat strategy, magic, and ninjutsu would make them a force to be reckoned with.
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.
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.