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.
If we're talking about a Halo and Dead Space crossover fanfiction, the Arbiter from Halo could also be a strong contender for the most powerful character. He has his own set of skills, combat experience, and leadership abilities. In a crossover, he could use his strategic mind to deal with the threats from Dead Space. However, we also have to consider the potential of a combined entity. For example, if a Necromorph was somehow fused with a Covenant Elite, the resulting being could be extremely powerful and unpredictable. So, it really depends on the creative twists and turns in the fanfiction.
However, it could also be a new character created specifically for the crossover. This character could have the best traits of both the Doom Slayer and the Master Chief. Maybe they have the Doom Slayer's combat skills and the Master Chief's understanding of technology. They could be a wildcard that changes the power dynamics in the crossover fanfiction.
It could be the Emperor of Mankind from Warhammer 40k. His psychic powers are off the charts and his influence over the entire Imperium is vast. He could easily manipulate the situation in a crossover. However, Master Chief from Halo also has his own set of skills and experience that make him a formidable opponent.
I think Cortana and Michonne would be an interesting pairing. Cortana has all this knowledge and intelligence, while Michonne is a fierce fighter. Cortana could provide strategic advice to Michonne, and Michonne could teach Cortana about the realities of survival in a world full of the undead. They could form a really unique bond.
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.
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.
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.
The most powerful character in a Star Wars Halo crossover fanfic could be different depending on the situation. If it's a battle of brute strength and combat skills, Master Chief would be up there. But when it comes to the power of the mind and manipulation, characters like Emperor Palpatine have an edge. However, if we consider the potential for growth and the ability to inspire others, a character like Leia Organa could be very powerful. She has leadership skills and is also Force - sensitive, which could be a deadly combination when combined with Halo technology or allies.
It's hard to say, but Thor could be a really powerful character in a Halo x Marvel crossover. He has his godly powers, like control over lightning and super strength. In the Halo universe, not many things can stand up to his power. He could easily take on Covenant forces or even some of the Flood with his hammer Mjolnir.
I think it could also be a character like Itachi. His Sharingan gives him incredible combat abilities such as genjutsu and enhanced perception. If he were to get his hands on Halo technology, he could use it in very strategic ways. For instance, using a cloaking device to enhance his already stealthy nature from his ninja skills. And his intelligence would make him a force to be reckoned with in any 'naruto halo fanfiction' scenario.
It could be Master Chief if he got access to Naruto's chakra. He's already a super - soldier, and with chakra, he could become even more powerful. He could use chakra to enhance his already amazing physical abilities and his combat skills with the various Halo weapons.