Harry Potter himself would be a great character in this fanfiction. He could bring his sense of justice and courage to Westeros. He might get involved in the political intrigue there, using his magic to help the oppressed. He could also introduce the concept of the wizarding world to the people of Westeros, which would completely change the power dynamics.
I think Tyrion Lannister would be very interesting. He's already a clever character in Game of Thrones. In a fanfiction with Harry Potter, he could learn magic and use it in his devious plans. He might even go to Hogwarts to study magic and cause all kinds of chaos there.
Definitely Daenerys Targaryen. She has her dragons in Game of Thrones. In a crossover fanfiction, she could meet with the magical community of Harry Potter. Maybe she would try to use magic to control her dragons better or learn about different forms of magic that could help her claim the Iron Throne.
Jon Snow would be an interesting character in a 'Game of Thrones' and 'Harry Potter' crossover. He has a strong sense of duty and honor, similar to some of the characters in Harry Potter. He could be introduced to the concept of magic and might be skeptical at first. But as he sees the power of magic, he could use it to better protect the North. He could also learn from the wizards about different ways of leadership. And his relationship with the direwolves could be explored in new ways, especially if there was some sort of magical connection between them and the magical creatures in Harry Potter's world.
However, Voldemort also has a claim to being the most powerful. His dark magic is extremely potent, and if he were to enter the Game of Thrones world, he could use fear and intimidation to gain power. He might even form alliances with some of the more ruthless characters in Westeros to achieve his goals.
Definitely Harry Potter himself. His ability to stand up to great evil would be interesting in the cutthroat world of Game of Thrones. He could be a force for good against the likes of Joffrey Baratheon or Ramsay Bolton.
Clark Kent and Arya Stark would be an interesting pairing. Clark with his superpowers and noble nature, and Arya with her assassin skills and strong will. They could learn a lot from each other. Clark could teach Arya about using her powers (if she were to gain some in this fanfic) for good, and Arya could teach Clark some of her stealth and combat techniques.
Reincarnating as Tyrion Lannister would be really interesting. He's already a complex character with a sharp mind. As someone reincarnated, you could use your modern ideas to enhance his political acumen even more. You could also try to change his relationship with his family for the better, or at least make his life less miserable while still being the clever dwarf everyone fears and respects in the political arena.
Black Widow. She's a master spy and assassin. In the world of Game of Thrones, she could easily blend in with the various spies and informants. She could play the game of thrones better than most of the characters there, using her charm and combat skills to manipulate the situation to her advantage. She might even be able to uncover some of the deepest secrets of the Seven Kingdoms.
Jon Snow would also be a great addition. His journey from being a bastard to a leader in Game of Thrones could be paralleled in the world of Frozen. He could learn from Elsa about controlling his powers (if we assume he has some new ice - related powers in this crossover) and in turn, teach the people of Arendelle about loyalty and sacrifice, which are values he has held dear in Westeros.
I think Arya Stark would be really interesting. In life, she was a fierce assassin. In the afterlife, she could use her skills in different ways. Maybe she's on a mission to hunt down the spirits of those who wronged her family. Or she could be teaching other souls self - defense in this new realm.
Arya Stark in a 'Naruto Game of Thrones crossover fanfiction' would be fascinating. If she were to learn some of Naruto's ninja skills, she would become an even deadlier assassin. And Naruto's influence might change her path a bit, making her less vengeful and more about bringing real change to Westeros.
Harry Potter and Jon Snow would be a very interesting pair. Harry is used to fighting against dark magic and Jon is a natural leader who has faced many challenges in Westeros. They could learn a great deal from each other. Harry could teach Jon about magic and Jon could teach Harry about leading in a world without magic but full of political scheming.