Well, it's really hard to say. The Primarchs in Warhammer 40k are incredibly strong, both physically and as leaders. They have superhuman abilities and are masters of warfare. But on the Halo side, the Librarians with their powerful psychic - like abilities in the form of using the Forerunner technology could also be a strong contender. It really depends on how the crossover is written and what powers are emphasized for each character.
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.
In my opinion, the Forerunners from Halo might be the most powerful. They had such advanced technology that they could reshape entire planets. In a crossover with Warhammer 40k, they could potentially outwit and overpower many of the factions there. But then again, the Chaos Gods in Warhammer 40k are also extremely powerful, with the ability to corrupt and destroy on a massive scale.
In Halo Warhammer 40k crossover fanfiction, the Librarians from Warhammer 40k are quite popular. Their psychic powers can lead to some really cool interactions with Halo's technology and characters. They can sense things in the Warp, and how would that work when dealing with the more scientific Halo universe? Cortana from Halo is also a favorite. Her artificial intelligence is something that doesn't really exist in the same way in Warhammer 40k. So seeing her interact with the various tech - priests or even daemon - possessed entities could be really interesting. Additionally, Chaos Space Marines are often used for their menacing and unpredictable nature in crossovers.
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'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.
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.
Chief Master Sergeant John - 117 from Halo is often a popular character in these crossovers. His super - soldier abilities and leadership make him a natural fit in the chaotic and dangerous Warhammer 40k universe.
Some interesting concepts are the different power sources. In Halo, we have things like plasma and fusion power, while Warhammer 40k has the Warp and its associated powers. How would these interact? Maybe a UNSC ship powered by a fusion reactor has to deal with the unpredictable effects of the Warp when it enters a region influenced by it. And then there's the concept of heroism. Spartans are known for their individual heroics, while in Warhammer 40k, heroism often comes with a heavy price and a lot of religious and ideological baggage. Combining these two views of heroism can make for some thought - provoking stories.
Some interesting concepts include the combination of the different power systems. In Halo, you have the MJOLNIR armor with its energy shields, and in Warhammer 40k, there are psykers with their psychic powers. Imagine a scenario where a Spartan with MJOLNIR armor has to face a Warhammer 40k psyker. How would their powers interact? Another concept is the cultural exchange between the two universes. The military hierarchies and honor codes in both settings could lead to some really complex and engaging storylines when combined.
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.
Ruby Rose is often a popular crossover character. Her combat skills and leadership in RWBY make her a natural fit to be inserted into the Warhammer 40K universe. She could potentially lead a squad of Space Marines or be a key figure in a resistance against some of the darker forces. Another popular one is Yang Xiao Long. Her strength and brawler - like combat style can be really interesting when pitted against the various foes in 40K.