It's really hard to say. Naruto has the power of the Nine - Tailed Fox and his own ninja skills which are extremely powerful. But in High School DxD, Great Red is an extremely powerful being. However, if Naruto could somehow combine his chakras with the magic in High School DxD's world, he might become even more powerful. So it could be Naruto if he adapts well to the new world.
It could be Issei from High School DxD if he were to gain Naruto's abilities. His already formidable power with the Boosted Gear, combined with Naruto's jutsus like the Rasengan or even the ability to use chakra, would make him extremely powerful. He could use chakra to enhance his physical abilities even more during his devil forms.
Jiraiya would also be a great character in this fanfiction. His pervy yet wise nature combined with his knowledge of senjutsu could lead to some really funny and also some very deep moments in the story. He could interact with the characters in High School DxD in a unique way, perhaps teaching them some of his own skills or learning from them as well.
Gohan and Issei would be really interesting. Issei is all about being a pervert and getting stronger in his own way. Gohan is this powerful yet humble Saiyan. Their different outlooks on power and relationships would create a lot of humorous and thought - provoking moments.
Ben and Issei would be an interesting pairing. Ben is used to being a hero with his Omnitrix, and Issei has his own journey of becoming a powerful devil. They could learn from each other's combat styles. Ben could show Issei how to use his alien forms' unique abilities in battle, and Issei could teach Ben about the world of devils and the importance of relationships in his world.
The Doom Slayer and Vali would make for an exciting interaction. Vali is a very powerful and ambitious character in High School DxD. He would likely see the Doom Slayer as a rival or a potential ally in his quest for power. The two of them could engage in some epic battles or team up against a common enemy. Additionally, the interaction between the Doom Slayer and Koneko would be interesting. Koneko is a quiet and strong - willed character, and she might be curious about the Slayer's motives and his unwavering dedication to his mission.
Sasuke might also be a top contender. His Sharingan and Rinnegan abilities could be enhanced by DxD magic. He could use his intelligence and combat skills along with newfound powers to become very powerful. For example, if he got a Sacred Gear that enhanced his genjutsu abilities, it would be nearly unbeatable.
It's really hard to say. Harry Potter has his powerful magic from Hogwarts and his connection to Voldemort which gives him a certain power boost. But in the DxD world, characters like Great Red are extremely powerful. If Harry was able to combine his magic with the power of a Sacred Gear, he could potentially be a very strong force. However, characters like Sirzechs Lucifer also have a vast amount of power. So it could be a toss - up depending on how the crossover is written.
Naruto Uzumaki is often a very popular crossover character. His status as a powerful ninja and his unique personality make him stand out. Fans like to see how he would interact with the characters from High School DxD.
Issei from High School DxD is also popular in crossovers. His pervy nature could create some humorous situations when interacting with the more serious Naruto characters. Also, his power as a devil and his ability to boost his power through his perverted desires can be an interesting contrast to the chakra - based powers in Naruto.
One possible fanfiction could be about Naruto characters attending the school in High School DxD. For example, Naruto's positive and determined attitude might clash with the more complex social and power dynamics in the DxD world. He could make new friends or even rivals among the devils, angels, and fallen angels.