I think Daenerys Targaryen would be really interesting. If she were reincarnated, she could use her past experiences to be an even more powerful and just ruler. She already had a complex journey in the original series, and reincarnation would add more depth to her character. She might be able to avoid some of the mistakes she made in her first life, like trusting the wrong people.
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.
Definitely Jaime Lannister. His character had a lot of development in the original series. If reincarnated, he could either redeem himself even further or take a completely different path. He might try to make amends for his past actions by becoming a true knight, or he could turn against his family if he realizes the toxicity of their ways. His reincarnation would bring a whole new set of possibilities to the story.
Jon Snow is quite popular. People like to be reincarnated as him because he has a lot of potential for growth and a mysterious backstory. They can use their new knowledge to perhaps avoid some of his tragedies and rise to power more quickly.
Stormbreaker itself could be considered a character in a sense. If it were to appear in the Game of Thrones world, it would be an object of great power. Many characters would vie for it. It could end up in the hands of someone like Daenerys, who already has a connection to powerful and magical things. This would make her even more of a dominant force in the story.
Jon Snow is quite popular. Given his complex backstory and his role in the original series, seeing him reincarnated in different scenarios is fascinating. People like to explore what would happen if he had a second chance at life with different knowledge or abilities. Daenerys Targaryen is also a common choice. Her journey as the 'Mother of Dragons' makes her a prime candidate for reincarnation stories, perhaps to correct the mistakes she made in her first life.
Cersei could be the most interesting in a sense. If she were reincarnated, she could either double down on her power - hungry ways or completely change. Maybe she could learn from her past mistakes and become a more benevolent ruler. Or she could use her reincarnation as a way to exact revenge on those who opposed her in her previous life in more devious and calculated ways.
Another interesting crossover would be between Tyrion Lannister and Dandelion. Tyrion is known for his wits and political acumen, while Dandelion is a bard full of charm and stories. They could have some really interesting conversations. Maybe Dandelion would sing tales of Tyrion's deeds, or Tyrion could use Dandelion to spread certain rumors for his political gain.
Corvo Attano would be a great choice. His stealth skills and combat abilities would make him a formidable force in Westeros. He could easily blend into the shadows of King's Landing or any other city in the Seven Kingdoms. His ability to take down multiple enemies silently would give him an edge in the often - violent and politically charged situations in 'Game of Thrones'.
Daenerys Targaryen would be fascinating. She has her own dragons, but Lightbringer could be the key to truly uniting the people of Westeros under her rule. It could symbolize her rightful claim to the Iron Throne and be a tool to overcome any remaining opposition.
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.