However, it could also be someone like Madara Uchiha if he were to be part of this crossover. His vast power, knowledge of jutsu, and his ability to control the Nine - Tailed Fox would make him a formidable force. He could easily decimate hordes of zombies and manipulate the situation to his advantage, even in the unfamiliar highschool setting.
It would probably be Naruto. His ability to summon large amounts of chakra and perform powerful jutsus like the Rasengan and Sage Mode would make him extremely formidable against zombies. He could also use his chakra to sense the presence of the undead from a distance, giving him an edge in survival.
It could be Issei from High School DxD if he were to gain Naruto's abilities. His already formidable power with the Boosted Gear, combined with Naruto's jutsus like the Rasengan or even the ability to use chakra, would make him extremely powerful. He could use chakra to enhance his physical abilities even more during his devil forms.
It's likely that Master Chief could be considered one of the most powerful. His combat training, physical enhancements, and access to advanced UNSC technology give him an edge. He has experience fighting large - scale battles and powerful alien threats. However, Isaac Clarke should not be underestimated. His knowledge of engineering and ability to survive against the Necromorphs in the Dead Space universe is remarkable. In a crossover, his skills could be adapted to deal with Halo - related threats as well.
I think Alice could be a very powerful character in the crossover. She can see the future, which would give her group a huge advantage. They could avoid dangerous situations with walkers or other threats before they even happen. And her speed and fighting skills as a vampire would also make her a force to be reckoned with.
It's really hard to say. Naruto in his strongest form, like when he has the power of all the tailed beasts, is extremely powerful. He has incredible strength, speed, and the ability to use a vast array of jutsu. However, Aang in the Avatar state has access to all four elements and can be a formidable opponent. If we consider the Avatar state's spiritual connection, it could give Aang an edge in a battle of wills.
Sesshomaru could also be a very powerful character in this crossover. He has his own powerful sword, Bakusaiga, and his inhuman strength and speed. He is very experienced in combat against demons. Compared to Naruto's friends like Sakura, Sesshomaru would likely be much more powerful in terms of raw strength and combat ability. But Sakura has her own medical ninjutsu and superhuman strength when she releases her inner power, which could also be useful in different situations.
It's likely Naruto himself. His ability to create multiple clones, his superhuman strength and speed from his training as a ninja, and his rasengan attack would be extremely effective against the walkers. He could easily take out large groups of them in no time.
I think Hermione would be really interesting. She's so intelligent and resourceful in the Harry Potter world. In Highschool of the Dead, she could use her vast knowledge of magic to come up with unique solutions to deal with the zombies. She might even find a way to communicate with them or understand their 'nature' through her research skills.
Sasuke would also be very powerful. His Sharingan gives him enhanced perception and the ability to copy moves. In the SAO world, he could quickly analyze the fighting styles of opponents and come up with counter - strategies. His Chidori and other lightning - based jutsu could be very effective against the digital enemies in SAO.
However, we can't forget about Steve. He has been living in the Minecraft world for a long time. He has in - depth knowledge of how to build the most powerful weapons and structures. He can use his pickaxe with great skill, which could be as effective as a ninjutsu in some cases. Also, his ability to survive in the Minecraft world's harshest conditions gives him an edge over the Naruto characters who are new to this environment.