Arceus would likely be the most powerful in terms of pure Pokemon power. It's a god - like Pokemon with the ability to create the entire Pokemon universe. However, if we consider Naruto characters as well, Naruto in his Six Paths Sage Mode could be extremely powerful. He has access to vast amounts of chakra and can perform incredibly powerful jutsus. If he were to team up with Arceus, they would be an unbeatable force.
Mewtwo is another very powerful Pokemon. It has psychic powers that are off the charts. In a fanfiction, if it were to be enhanced by some Naruto - like chakra or techniques, it could become even more powerful. On the Naruto side, Kaguya could also be a powerful entity. She has the ability to create dimensions and has a vast amount of power. If Mewtwo and Kaguya were to face off, it would be a battle of epic proportions, and it's really difficult to determine who would be the ultimate winner in this mash - up of universes.
In a 'Naruto Pokemon Sun and Moon' fanfiction, Ultra Necrozma could also be a top contender for the most powerful. It has the ability to absorb light and is extremely powerful in battle. But then again, characters like Sasuke with his Rinnegan could potentially match up against it. Sasuke can teleport, has powerful fire - based attacks, and his Sharingan gives him great combat awareness. So it's hard to say who would be the most powerful when you combine the two universes.
Definitely Naruto himself. He has such a vibrant and unique personality. His never - give - up attitude would make him an amazing Pokemon trainer. He would approach Pokemon battles with the same tenacity he has in ninja battles. Also, his ability to form strong bonds, like he did with his friends in the Leaf Village, would translate well into bonding with his Pokemon.
Lillie is often a popular character. In fanfiction, she might be given more in - depth storylines, such as her further exploration of her connection with the Legendary Pokemon. Maybe she discovers a new form of Nebby or has an adventure that reveals more about her family's past.
Gladion is also a top choice. His edgy personality and his connection to the Aether Foundation and the Ultra Beasts offer a lot of potential for fan - created stories. You could write about his secret missions or his past experiences that shaped him into the person he is in the game.
Gladion is also a favorite. His cool and mysterious demeanor, along with his strong Pokemon team, attracts a lot of fanfiction writers. They can create stories where Gladion teams up with other characters or has his own solo adventures that reveal more about his character and his goals in the Pokemon world.
One interesting plot could be Naruto getting transported to the Pokemon Sun and Moon world and having to learn how to be a Pokemon trainer. It would be cool to see how his ninja skills translate into the Pokemon universe.
I think it could be Naruto in his most powerful form, like when he has all the tailed beasts' chakra and is in his ultimate Sage Mode. His attacks are extremely destructive and he has a lot of combat experience. However, Sailor Saturn is also a very powerful character in the Sailor Moon universe. She has the power of destruction which is on a large scale. If they were to fight, it would be interesting to see how Naruto's positive energy and fighting skills would fare against Sailor Saturn's more passive yet extremely powerful destruction abilities. But overall, Naruto's versatility and his growth potential might make him the most powerful in a crossover scenario.
A possible plot is that the Akatsuki from Naruto are after some powerful and rare Pokemon in the Alola region. They believe these Pokemon can give them ultimate power. The Alola trainers, along with Naruto and his team, have to stop them. Naruto could bond with a Pikachu that has some unique abilities, similar to how he bonded with Kurama. They face off against the Akatsuki in epic battles where both ninja techniques and Pokemon moves are used.
Gladion is also quite popular. His cool and mysterious demeanor, along with his strong Pokemon battling skills, draw a lot of attention. Fanfic writers might write about his past, his relationship with his sister Lillie, or his battles against powerful opponents.
Definitely Naruto himself. He has an unwavering will and determination. If he had a Pokemon like Charizard, their combined strength would be incredible. Naruto's ability to make friends and form strong bonds could also make his relationship with his Pokemon very deep. He could use his ninjutsu in combination with his Pokemon's attacks, for example, using his Rasengan while his Pokemon distracts the enemy.
One popular type of OC is the 'chosen one' character. This is an OC who is prophesied to save the Alola region. They often have a special power or pokemon that sets them apart. For example, an OC with a psychic ability that allows them to communicate with all pokemon. This makes them a key figure in any story where the Alola region is in danger.