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.
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.
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.
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.
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.
In a Naruto - High School DxD fanfiction, the power levels would be really interesting to explore. Naruto's sage mode and tailed - beast forms are extremely powerful. In the DxD world, there are different power systems like the Sacred Gears. It would be fascinating to see how Naruto's powers interact with the powers of the DxD characters. For example, could his Rasengan be enhanced by a Sacred Gear? Or how would he fare against a devil's magic?
One possible scenario could be that Naruto characters are somehow transported to the world of High School DxD. Naruto's ninjutsu skills would be quite a shock to the devils and angels in that world. For example, Sasuke's Sharingan could give him an edge in battles against the supernatural beings in High School DxD. And Naruto's never - give - up attitude might inspire some of the characters there to face their challenges more bravely.
Sesshomaru could also be a very powerful character in this crossover. He has his own powerful sword, Bakusaiga, and his inhuman strength and speed. He is very experienced in combat against demons. Compared to Naruto's friends like Sakura, Sesshomaru would likely be much more powerful in terms of raw strength and combat ability. But Sakura has her own medical ninjutsu and superhuman strength when she releases her inner power, which could also be useful in different situations.
It's hard to say for sure, but Alucard has the potential to be extremely powerful. His vampiric abilities like regeneration, immortality, and the ability to summon his army of familiars give him a huge edge. He can take a lot of damage and keep coming back. In a fight against Naruto's ninjutsu or the various powerful beings in the DxD world, he could be a force to be reckoned with.