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.
Ichigo from 'Bleach' could be a very powerful character in the crossover. He already has strong Shinigami powers, Hollow powers, and Quincy powers. If he were to gain Sekirei - like abilities on top of that, he would be extremely formidable. His ability to adapt and grow quickly would make him a force to be reckoned with.
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.
But we also can't forget about Ichigo in his final form. He has the power of a Soul King and his combat skills are top - notch. If he were to enter the Fairy Tail world with all his power, he would be able to take on most threats with ease. It really depends on how the crossover is written and what kind of challenges are presented to each character.
It's hard to say for sure, but Ichigo could be a very powerful character in this crossover. He has his Hollow powers, his Soul Reaper powers, and his Quincy powers all combined. In a fight against the powerful characters from Soul Eater, like Asura, his multiple power sources would give him an edge. He can quickly adapt to different situations and opponents, and his high - speed attacks and strong spiritual pressure would be difficult for many to handle.
It's hard to say for sure. Ichigo in his final form in Bleach has extremely high power levels. He can cut through almost anything with his sword and has great speed and strength. However, in Fairy Tail, Acnologia is a very powerful dragon slayer. His dragon form gives him incredible power and he has a lot of combat experience. So it could be either of them depending on how the fight is set up.
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.
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.
The antagonists in 'bleach the gamer fanfic' can also be popular. If they are written in a way that challenges the gamer character's skills and strategies, they become memorable. Maybe an antagonist who can also adapt to the gamer - like tactics of the main character, creating a tense and exciting back - and - forth in the story.
Rukia is also popular. Her connection to Ichigo and her own unique powers and backstory can be explored in new and exciting ways in reincarnation fanfics. For example, she might be reincarnated in a different part of the Soul Society with a new role to play.