Definitely Naruto himself. His positive attitude and never - give - up spirit would contrast greatly with the more morally ambiguous characters in Highschool DxD. He could also introduce the concept of chakra and ninja techniques, which would be completely new to the Highschool DxD world. His rasengan, for example, would be a very powerful and unique ability in that universe.
Rias from Highschool DxD would be a great crossover character into the Naruto world. She's a strong and intelligent devil. In the Naruto world, she could use her devilish charm and strategic mind to navigate the complex politics of the ninja villages. Her powers of teleportation and the ability to create demonic servants would make her a formidable force, and it would be interesting to see how she interacts with Naruto's friends and enemies.
I think Naruto would be the most interesting. His energetic and determined personality would stand out in the Highschool DxD world. He could bring his ninja way of life and values, which are quite different from the devilish and angelic cultures in DxD. His ability to make friends easily might also lead to some really unique interactions with the DxD characters.
Naruto and Rias would be really interesting. Naruto's straightforward and determined personality combined with Rias' leadership and strategic mind could lead to some great interactions. They could learn from each other's fighting styles and ways of dealing with problems in their respective worlds.
Naruto himself would be a great choice. His sunny personality and strong will would contrast with the more complex and often morally ambiguous characters in Highschool DxD. He could bring a sense of innocence and pure determination to the world of DxD, and his unique jutsu would be a new and exciting addition to the battles there.
Definitely Naruto and Issei. Issei is all about his perverted ways and getting stronger, while Naruto is focused on protecting his friends and achieving his dreams. Their clash of personalities would be really interesting. Naruto might not understand Issei's obsession with girls at first and try to set him straight. And Issei would be amazed by Naruto's power and want to learn from him.
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.
Sakura could be an interesting crossover character. In Highschool DxD, she could show off her medical skills which are quite advanced in Naruto. She could also develop her combat skills further by learning from the DxD characters. Her strong - willed personality would make her a force to be reckoned with, and she could get involved in the peerage system, perhaps even leading her own peerage.
Natsu and Akeno could have some fun interactions. Akeno has her own brand of mischief, and Natsu might not be used to her teasing ways. Their first meeting could be full of misunderstandings, with Natsu thinking she's an enemy at first because of her devious appearance, but then realizing she's an ally and developing an interesting friendship.
Itachi and Rias would be really interesting. Rias is used to being in control and having power, but Itachi's calm and collected nature along with his overwhelming power would put her in a different position. She would likely be intrigued by his Sharingan and his combat abilities. Itachi, on the other hand, would be dealing with a different kind of power structure and would have to navigate the social hierarchy of the devils in Highschool DxD.
Ichigo and Akeno could also be an exciting pairing. Akeno has her own unique powers and a mysterious side. Ichigo's straightforwardness and strong moral code could contrast with Akeno's more complex nature. They could work together in battles, with Ichigo providing the brute force and Akeno using her strategic and powerful attacks.
Ichigo and Rias would be a very interesting pairing. Ichigo is a powerful and determined fighter, and Rias is a strong - willed Devil. Their personalities could clash at first, but then they might find common ground in their leadership roles and their desire to protect their respective groups.