Whitebeard from One Piece could be a very powerful character in a crossover. His Tremor - Tremor Fruit is already one of the most destructive powers. If he were to gain some Hollow - like powers that enhanced his strength and durability even further, he would be a terrifying opponent. His leadership skills and the loyalty of his crew would also give him an edge in any situation.
Ichigo could also be a strong contender. His Hollow and Soul Reaper powers are already very powerful. If he were to get his hands on a Devil Fruit, say the Logia type that allows him to turn into an element, he could become even more powerful. He has the experience of fighting powerful enemies in both the Bleach world and could easily adapt to the challenges in the One Piece world.
It's a tough call, but it could be Luffy. His Devil Fruit powers combined with his indomitable will make him extremely formidable. If he were to somehow gain some Soul Reaper abilities on top of that, like the ability to use a Zanpakuto, he would be near - unbeatable. His Gear Fourth form already gives him enhanced strength and speed, and with Soul Reaper powers, he could be a force to be reckoned with.
Ichigo and Luffy would be a really interesting pairing. Ichigo is all about protecting his friends and family and has a strong sense of justice. Luffy is the same, always looking out for his crew. They both have powerful and unique fighting styles. Ichigo's swordsmanship and Hollow - influenced powers would contrast with Luffy's Devil Fruit - based stretching abilities. They could learn a lot from each other in terms of combat and leadership.
I think it could be a toss - up between Goku and Whitebeard. Goku has his incredible ki attacks and speed, but Whitebeard had the power to cause earthquakes and was considered one of the strongest pirates in the One Piece world. If Whitebeard were to face Goku, his experience and the power of his Devil Fruit could potentially give him an advantage, especially if he caught Goku off - guard.
It's really hard to say. Luffy has his Gear Fourth form and the ability to stretch his body in incredible ways. But Naruto in his Sage of Six Paths mode is extremely powerful too. He has access to a vast amount of chakra and can use some of the most powerful jutsu. However, if we consider Kaido from One Piece, with his near - indestructible body, he would also be a top contender. So it could be any of them depending on how the crossover is written.
It's hard to say for sure. Luffy has the power of the Gum - Gum Fruit and his Haki, which gives him great strength and the ability to sense and overpower opponents. However, Naruto has the Nine - Tails' chakra and Sage Mode. In a crossover, if they were to fight, it might depend on the situation. But if we consider all - round power, Naruto's ability to create multiple clones and use a variety of jutsu might give him an edge over Luffy in some scenarios.
It's really hard to say who would be the most powerful. Saitama from One Punch Man (if we were to include him in this wild crossover thought experiment) would probably be a top contender just because he can defeat any opponent with one punch. But in a more traditional Bleach - Marvel crossover, if the Living Tribunal from Marvel was involved, it would be extremely powerful. It has near - omnipotent powers and oversees the entire Marvel multiverse. However, in Bleach, Yhwach has extremely powerful abilities as well, like the ability to see the future and rewrite it. So it really depends on the context and how the crossover is set up.
In my opinion, Aizen could be one of the most powerful characters in such a crossover. His intelligence and his ability to manipulate others are already very dangerous in the Bleach universe. If he were to enter the Inuyasha world, he could easily use his wits to turn the different factions against each other. He could also potentially find a way to use the demon energy for his own gain. His Zanpakuto's powers are also very versatile and could give him an edge against the characters in Inuyasha's world.
It's likely to be Goku. His ability to constantly increase his power level through training and reaching new forms like Super Saiyan God and Ultra Instinct gives him an edge. He could potentially overpower most of the Bleach characters.
It's really hard to say. Luffy has his rubber powers and Haki which can be extremely powerful. But Naruto has the Nine - Tailed Fox and his vast array of jutsu. If they combined their powers, they would be a force to be reckoned with. However, if we consider villains, someone like Madara Uchiha with his Sharingan and all his skills, if he got his hands on a Devil Fruit, he could be extremely dangerous. But then again, Kaido from One Piece is also super tough. So, it's a tough call.
I think it might be a toss - up between Goku (just kidding, not really part of the crossover but for comparison). Seriously, though, it could be Kaguya from Naruto. Her powers are almost god - like with the ability to create dimensions and control time. In the One Piece world, there's not really anyone who can directly counter those abilities. But then again, Im from One Piece has a lot of mystery around him and could potentially be extremely powerful. If Im had some way to nullify chakra, he could be the most powerful in a crossover situation.
Dumbledore might be the most powerful. He has vast magical knowledge and experience. In a crossover, he could quickly understand the mechanics of Devil Fruits and use his magic to counteract any threats from One Piece characters. He could also use his intelligence to outwit the pirates and use spells like the Patronus to protect himself and others from danger.