One of the most powerful characters could be a combination of characters. For example, if Gray from Fairy Tail and Sasuke from Naruto combined their powers. Gray's ice - make magic and Sasuke's Sharingan and Rinnegan abilities could create a very powerful and unique set of skills. Gray could use his ice to freeze opponents while Sasuke uses his visual powers to predict their moves and counterattack. Also, if they could inherit some of the immortality aspects from either universe, they would be even more difficult to defeat.
I think it might be a toss - up between Zeref from Fairy Tail and Kaguya from Naruto. Zeref has his immortality and incredibly powerful dark magic. Kaguya, on the other hand, has god - like powers in the Naruto universe. If they were in the same universe and could access each other's powers or knowledge, the resulting combination would be terrifyingly powerful. For example, if Zeref could use Kaguya's ability to create dimensions, or if Kaguya could use Zeref's death - dealing magic, they could become the most powerful being.
It could be Naruto himself. With his already powerful ninjutsu and the potential to gain Fairy Tail magic, he would be extremely formidable. His ability to quickly learn new skills and his large chakra reserve, combined with something like Fairy Tail's celestial magic, would make him a force to be reckoned with.
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 could be Naruto if he combines his chakra powers with Fairy Tail magic. He has an enormous amount of chakra and his ability to learn new things quickly could make him a formidable force. He might be able to master powerful magic spells on top of his already strong ninjutsu.
Rikuo Nura from Nura Rise of the Yokai Clan has a strong claim. He is the leader of the Nura clan and has both yokai and human forms. His yokai powers are vast, and if he were to gain some ninja skills from the Naruto universe, he could be nearly unstoppable. He has the ability to command a large number of yokai, which is a great asset in any battle.
It could be Naruto himself. He has an enormous amount of Chakra and his ability to learn new techniques quickly. If he were to partner with a powerful Digimon like Omnimon, he would be nearly unstoppable. His ninja skills combined with Omnimon's power would make him a force to be reckoned with.
I think it could be a combined form of a character. For example, if Issei were to somehow gain the power of the Nine - Tailed Fox. He already has his Boosted Gear and his experience in battles in the Highschool DxD world. Adding the Nine - Tailed Fox's chakra would make him an extremely formidable opponent. He could use the fox's chakra to enhance his Sacred Gear even further, and his new form might have abilities that are a combination of both universes, like using the fox's tails as a weapon while also having the power of the Boosted Gear.
It's really hard to say. Goku in his Super Saiyan Blue form has incredible power and can destroy planets. But Naruto in his Six Paths Sage Mode also has amazing abilities. However, if we consider raw power, Goku might be more powerful as he can reach levels of power that can destroy entire solar systems. But Naruto's intelligence and his ability to strategize could give him an edge in a fight.
It's really hard to say. Naruto has some extremely powerful characters like Naruto himself with his Nine - Tails chakra and Sage Mode. But in Yu Yu Hakusho, characters like Yusuke at his Mazoku form are also incredibly strong. It might depend on how the crossover is written. If the power - up systems are combined in a certain way, it could be a character who manages to blend both types of powers most effectively.
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.
One way could be through a crossover spell or device. For example, in fanfiction, a powerful mage from Fairy Tail might accidentally create a portal that leads to the Naruto universe while trying to perform a complex teleportation spell. The White Dragon could then find itself in the Naruto world, leading to all sorts of interesting interactions. Maybe the White Dragon's power is mistaken for a new type of chakra - based ability, and the ninja start to study it.
One way could be through a magical portal. Maybe some Fairy Tail magic creates a portal that sucks Naruto and his ninja friends into the Fairy Tail world. Then they have to team up with the Fairy Tail mages to face a common enemy.