If we consider strategy and intelligence along with power, Levi from Attack on Titan could be a top contender. His incredible combat skills and quick thinking could make him a force to be reckoned with even among the superpowered heroes. But then there are heroes like Eraserhead who can nullify quirks, which could be a huge advantage against some of the more dangerous Attack on Titan characters.
It could be All For One from My Hero Academia if he managed to absorb the powers of some Halo characters. Imagine him having the strength and abilities of a Spartan on top of his already formidable power - stealing Quirk. He would be almost unstoppable.
A character like Kaido with a quirk that made his already tough body even more indestructible would be a nightmare to face. On the other hand, Deku with the ability to use all the One Piece Haki types in addition to his multiple quirks would be an absolute powerhouse. He could use Armament Haki to strengthen his punches even more, Observation Haki to predict his opponents' moves better, and Conqueror's Haki to intimidate his foes. In a fanfiction world, the possibilities for creating the most powerful character are endless.
Sasuke with a quirk could also be extremely powerful. If he had a quirk like Shoto Todoroki's ice and fire ability on top of his Sharingan and Rinnegan, he would be a force to be reckoned with. He could use his ice to freeze opponents from a distance, then use his fire to create powerful explosions, all while using his Sharingan to predict their moves and his Rinnegan for more advanced techniques.
It's hard to say for sure, but All Might combined with the Wizard King could be extremely powerful. All Might has his super strength and hero experience, and the Wizard King has incredible magic power and leadership skills. Together, they could take on any threat with ease.
Probably a combination of Deku and Ken. Deku's One For All power gives him incredible strength and speed, and Ken's Shoryuken and other moves would add a whole new level of combat ability. Deku could use Ken's fighting style to better control his power, and Ken could benefit from Deku's raw power. They could take on any enemy in the combined universes with ease.
Scorpion could also be a top - tier powerful character in this fanfiction combo. His hellfire abilities and his ninja skills are no joke. In a fight against the My Hero Academia characters, he could easily surprise them with his teleportation and his ability to throw his spear. However, someone like Endeavor with his powerful fire quirk could also put up a good fight against Scorpion.
Combining Doctor Strange and Shoto Todoroki could result in a very powerful character. Doctor Strange has his mastery over magic, with the ability to create portals, manipulate time, and use various spells. Shoto Todoroki has his ice and fire Quirks. The combination would mean that this new character could use ice and fire attacks while also being able to use magic to control the battlefield. They could freeze enemies and then shatter them with fire spells or use portals to surprise opponents.
In a fanfiction like this, there could be some interesting alliances. Eren from Attack on Titan, with his Titan - shifting ability, could team up with Deku. Deku could learn a lot from Eren's willpower and determination. Also, the quirks in My Hero Academia might be used to develop new strategies against the Titans. Like Todoroki could freeze the Titans' legs to stop them from moving.
If you combined Edward Elric and Eren Yeager, they would probably be very powerful. Edward has his alchemy skills which can be used for various things like creating weapons or barriers, and Eren has the ability to transform into a Titan. Together, they could use alchemy within the Titan form, perhaps creating stronger structures or weapons directly on the Titan's body.
Probably a combination of All Might and Gol D. Roger. All Might has incredible strength and the symbol of peace status. Roger was the Pirate King with great leadership and combat skills. Together, they would be almost unbeatable, having the best of both the hero and pirate worlds in terms of power and influence.