Who would be the most powerful character if Naruto and Avatar The Last Airbender universes collided in fanfiction?

It's hard to say for sure, but Naruto himself could be a very powerful contender. His vast array of jutsu and his ability to harness the Nine - Tails' power would make him extremely formidable. He could easily take on many opponents at Marineford with his shadow clones and powerful attacks like the Tailed Beast Bomb.

It could be Danny. His ghost powers give him a wide range of abilities like flight, intangibility, and ectoblasts. In the Digital World, these powers would be very useful. He could also phase through digital barriers and surprise his enemies. Also, his ability to overshadow people or Digimon could give him an edge in battles.

It's likely the Doom Slayer. He has proven to be an unstoppable force against hordes of demons. His strength, speed, and arsenal of weapons make him a formidable opponent. In a collision with Warhammer 40K, he could take on many of the threats there as well.

The Catalyst from Mass Effect might also be a very powerful entity in such a crossover. It has control over the Reapers and a vast understanding of synthetic and organic life. In a collision with the Terminator universe, it could potentially manipulate or even control the Terminators, as they are machines. It could use them to further its own goals, whatever those might be in this new combined scenario.

Kakashi Hatake could also be in the running. His Sharingan and vast array of ninja techniques make him a formidable opponent. In the Star Wars side, Ezra Bridger has a strong connection to the Force. But Kakashi has more combat experience and a more diverse set of skills. If they were to face off, it would be an interesting battle. And of course, Naruto's ability to inspire and bring out the best in others could also be considered a form of power, which could tip the scales in his favor in this colliding universes scenario.

If we consider the powers and abilities in both universes in a fanfiction context, I think it could be Eri from My Hero Academia. Her quirk has the potential to be extremely powerful as it can rewind a person's body to a previous state. If she could learn to control it fully, she could potentially 'undo' the powerful undead in the Overlord universe or even reverse the magic spells. However, this is a very speculative idea as she is still a young and inexperienced character, and the power of Ainz Ooal Gown and his allies in the Overlord world should not be underestimated.

Another strong contender could be Raiden. As a thunder god in Mortal Kombat, he has control over lightning and has great combat skills. In the Harry Potter universe, his lightning attacks could be very difficult to defend against. He could also use his powers to disrupt magic, as electricity can interfere with magical devices. His immortality also gives him an advantage, as he can keep coming back in a fight no matter how much damage he takes.

Magneto might also be extremely powerful in such a crossover. His ability to control metal could be a huge advantage in the wizarding world. He could manipulate the metal in wands, cauldrons, and other magical objects. However, he would have to deal with the unique magic that doesn't rely on physical elements all the time.

If we consider the character of Harry Potter himself, his ability to resist the Killing Curse and his connection to the Deathly Hallows could make him a formidable force. In the world of Bioshock Infinite, he could use his magic to protect the oppressed and fight against the tyrannical rule in Columbia. His courage and the power of love, which are important themes in his own story, could also play a significant role in this crossover scenario, perhaps even influencing the characters in Bioshock Infinite in unexpected ways.

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.