I think Mewtwo would be the most fascinating for Naruto to talk to. Mewtwo is a very intelligent and powerful Pokemon. It has complex thoughts about its existence and purpose. Naruto, with his own struggles and growth in the ninja world, could have deep philosophical discussions with Mewtwo. They could exchange ideas about power, identity, and what it means to be different in their respective worlds.
In such a fanfiction, Naruto might use his ninja skills to help Pokemon in battles. For example, he could use his shadow clone jutsu to confuse the opponents of a Pokemon. He could also communicate with Pokemon to understand their problems and help them find solutions. Maybe he would team up with Pikachu and use his Rasengan combined with Pikachu's thunderbolt for a super powerful attack.
Mewtwo is another favorite. Given its complex backstory and powerful abilities, in 'pokemon can talk fanfiction', it can have really profound conversations. It might discuss its feelings of being a created Pokemon, its views on the relationship between Pokemon and humans, and its search for its true place in the world.
One of the most interesting could be the ghost of the First Hokage. He has so much knowledge about the founding of the village and the early days of the ninja world. Naruto could learn a great deal from him, like how to better lead and protect the village.
Definitely Pikachu. Since Naruto is often in need of a quick and agile ally, Pikachu's speed and electric attacks would be very useful. Plus, Pikachu's cute appearance but powerful abilities would create an interesting contrast with Naruto's ninja style.
Sasuke could be a very interesting character in this crossover. He has his own dark and brooding nature. He might be more interested in the stronger and rarer Pokemon. His sharingan could give him an edge in understanding Pokemon's abilities and behaviors. He could also train Pokemon in a more intense and strict way compared to other trainers.
In this kind of fanfiction, Naruto is a must - have interesting character. His ability to make friends quickly could lead to some great relationships with different Pokemon. Then there's Jiraiya. He could be an interesting addition as he might be interested in studying the Pokemon and their abilities. He could use his toad summons in combination with Pokemon in battles. Also, the legendary Pokemon such as Rayquaza would be very engaging. It could be seen as a powerful and mysterious being that Naruto has to earn the trust of.
It's possible that Naruto would talk to the ghosts of the Uchiha clan members. Given the complex history of the Uchiha and Sasuke's importance in Naruto's life, communicating with these ghosts could help Naruto understand the clan better. He might also be able to prevent further tragedies related to the Uchiha by learning from their past mistakes.
I think Bulbasaur would be a good match. Naruto has a strong connection to nature in his own world, and Bulbasaur is a nature - based Pokemon. Naruto could use his knowledge of plants from his ninja training to help Bulbasaur grow stronger.
One interesting story could be about a Pikachu that uses its ability to talk to form an alliance with other Pokemon to save their forest from being cut down by humans. It convinces the local humans of the importance of the forest through its cute and intelligent communication.
Naruto would be the most interesting. His energetic and head - strong personality would make for great interactions with the Pokemon world. He could be constantly amazed by the different Pokemon and their abilities, yet still use his ninja skills and quick thinking to adapt. His ability to make friends easily would also mean he'd quickly bond with his Pokemon and other trainers.