I think it might be Xehanort. He has shown great power and cunning in Kingdom Hearts. If he were to gain the power of the aeons from Final Fantasy 10, like Bahamut or Shiva, he could become an unstoppable force. He could use his knowledge of the Heartless and Nobodies along with the aeons' powers to conquer both worlds in the fanfiction.
Possibly Ansem. He was already a very powerful figure in Kingdom Hearts. If he were to access the advanced magic and summons of Final Fantasy 10, he could be a dominant force. His understanding of the darkness and his ability to manipulate it, combined with the magic of Spira, would make him a formidable adversary or a very powerful ally depending on how the fanfiction is written.
It's hard to say for sure, but many would argue that Naruto could be extremely powerful. His ability to use chakra, perform various ninjutsu, and access the power of the Nine - Tails gives him a huge edge. However, characters like Thor with his godly powers and the Hulk with his incredible strength also can't be underestimated.
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.
Another possibility is a combination of all the Power Rangers morphing into their Mega - Zord form while Ben uses the Omnitrix to transform into a powerful alien like Way Big. The combined strength of the Mega - Zord and Way Big could be unrivaled. They would be a huge force, able to take on any villain that dares to challenge them, with the Zord's size and strength complementing Way Big's power and abilities.
Perhaps Naruto combined with Toph. Naruto's agility and combat skills along with Toph's earthbending mastery would be quite a combination. Toph could sense enemies from afar using her earthbending senses and Naruto could quickly engage them in combat. Also, Naruto could use his clones to help Toph with larger - scale earthbending projects, like building massive structures for defense or offense in a battle.
Sasuke could also be very powerful in this crossover. His Sharingan gives him an edge in combat, and if he were to form a bond with a Digimon, he could use his Sharingan to analyze and predict the enemy's moves better. Also, his ability to use different chakra natures could be combined with the Digimon's powers in unique ways. For example, using his Lightning chakra with an Electric - type Digimon to create super - charged attacks.
But we also can't forget about Arsene, Joker's initial Persona. Arsene has some powerful abilities on his own. In the combined fanfiction world, he could potentially possess some unique qualities from the No Game No Life world as well. Maybe he could gain the ability to manipulate the rules of games like the characters in No Game No Life do, making him an extremely formidable force.
In a fanfiction that combines these two universes, it really depends on how the powers are combined. If we consider pure raw power, perhaps a fusion between Goku and a Legendary Pokemon like Dialga would be the most powerful. Dialga can control time, and Goku has his Super Saiyan forms and vast combat experience. But if we look at strategic power, a trainer like Cynthia who has a deep understanding of Pokemon and somehow acquires the ability to use Ki could be extremely dangerous. She could outwit her opponents and use her Pokemon's and her own newfound powers to dominate in battles.
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.
Sora would be a very powerful contender. He has a lot of experience fighting all kinds of enemies in different worlds in Kingdom Hearts. His Keyblade skills and ability to use magic would give him an edge. Also, he has the power of the heart on his side. But Izuku Midoriya with One For All at its full potential could also be extremely powerful. He has incredible strength and speed, and his determination is unwavering.
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.