It's hard to say for sure, but Master Chief could be a strong contender. His combat training and advanced armor give him an edge. However, Darth Sidious is extremely powerful with his dark side abilities. He could potentially manipulate Master Chief if he got the chance. But then again, the Forerunners from Halo have such advanced technology that they might be able to counter the Force -powers. So it really depends on how the story is written.
In a Halo and Star Wars crossover fanfiction, the most powerful character could be a combination of both universes. For example, a character who has the combat skills of a Spartan and is also Force -sensitive. This character could use the Force to enhance their physical abilities even further, like using telekinesis to move faster or jump higher while in their Spartan armor. Or it could be a Forerunner who somehow gains the ability to use the Force. They already have incredibly advanced technology, and adding the Force to their arsenal would make them nearly unstoppable. However, it's also possible that a character like Luke Skywalker, if he had access to Halo technology, could become extremely powerful as well. He could use the Force to operate the advanced machinery and weapons from Halo, making him a force to be reckoned with in both universes.
If we consider the combination in fanfiction, Yoda could be very powerful. His deep connection to the Force and his wisdom make him a formidable force. But the Librarian from Halo also has vast knowledge and powerful abilities. She could potentially use her knowledge of the Halo rings and Forerunner technology to hold her own against the Jedi. And of course, the Emperor Palpatine has his own brand of power, being able to control the entire Galactic Empire. It's a complex comparison as the powers in both universes are so different.
I think Qrow could be really powerful in this crossover. He has his own unique abilities in RWBY, and if he were to also master the Force, especially the more combat - oriented aspects like telekinesis to throw his weapon with more force or use the Force to enhance his reflexes, he'd be a force to be reckoned with. He also has a lot of experience in combat which would help him adapt to the new powers.
It's hard to say for sure, but Ruby could be a very powerful character. Her speed and scythe skills are already formidable in the RWBY universe, and if she were to gain some of the combat knowledge from For Honor, like the strategic thinking of a For Honor general, she could be extremely powerful. She might be able to quickly analyze her opponents' fighting styles and counter them effectively.
It could be Naruto himself. His ability to tap into the Nine - Tails' chakra gives him an incredible power boost. In the merged universe, he might be able to use the Force as well, which would make him an unstoppable force. With his speed, strength, and now potentially the power of the Force, he could easily defeat most opponents.
Thor could also be very powerful. He has his Asgardian strength and his hammer (or Stormbreaker). If he were to also gain the ability to use magic like the wizards in Harry Potter, he could be near - invincible. He could call lightning while also casting spells. And his knowledge of different realms could give him an edge in any battles that span across the combined universes.
It could be Naruto himself. He has an enormous amount of Chakra and his ability to learn new techniques quickly. If he were to partner with a powerful Digimon like Omnimon, he would be nearly unstoppable. His ninja skills combined with Omnimon's power would make him a force to be reckoned with.
Gohan in his Ultimate form could also be a very powerful character in this combined fanfiction universe. He has incredible strength and intelligence in combat. But on the RWBY side, Ozpin has a lot of hidden power and knowledge. Still, when comparing their power levels, Gohan's physical strength and energy projection abilities are on a different scale. He could potentially take on even the toughest Grimm with ease. So, overall, Gohan might be among the most powerful in this crossover scenario.
I think it could be a combined form of a character. For example, if Issei were to somehow gain the power of the Nine - Tailed Fox. He already has his Boosted Gear and his experience in battles in the Highschool DxD world. Adding the Nine - Tailed Fox's chakra would make him an extremely formidable opponent. He could use the fox's chakra to enhance his Sacred Gear even further, and his new form might have abilities that are a combination of both universes, like using the fox's tails as a weapon while also having the power of the Boosted Gear.
However, some might argue that Levi Ackerman could hold his own. His incredible speed and combat proficiency with blades could potentially make him a match for some of the Star Wars characters. If he were to get his hands on a lightsaber, he might become an even greater threat. But in the grand scheme of things, when facing the more powerful Force - using characters, he might still be at a disadvantage.
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.
Perhaps Daenerys Targaryen. If she were to somehow acquire vampire - like abilities, she would be an even more formidable force. Her dragons combined with vampire powers such as enhanced senses and immortality (in a sense) would make her a very powerful and dangerous character. She could use her newfound powers to rule Westeros more effectively or to take on new enemies.