Aro from Twilight could also be a very powerful character in a combined universe. He has powerful mind - control abilities in the Twilight world. In the Game of Thrones universe, he could use these powers to manipulate the various lords and ladies, causing chaos or building his own power base. He could also use his vampiric strength and speed to defend himself against any threats, like the assassins in King's Landing or the wildlings.
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.
Arya Stark combined with Inuyasha could be extremely powerful. Arya has her assassin skills and ability to change faces, and when combined with Inuyasha's demon powers and combat skills, she would be a force to be reckoned with. She could use Inuyasha's speed and strength to take on even the toughest opponents in both worlds.
However, Voldemort also has a claim to being the most powerful. His dark magic is extremely potent, and if he were to enter the Game of Thrones world, he could use fear and intimidation to gain power. He might even form alliances with some of the more ruthless characters in Westeros to achieve his goals.
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.
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.
It could be Jon Snow. If he has a Stand in the fanfiction, combined with his already proven leadership skills and combat abilities, he would be a force to be reckoned with. His ability to come back from the dead might also be related to his Stand powers in some way, making him even more formidable.
It's really hard to say. Goku in his Super Saiyan Blue form has incredible power and can destroy planets. But Naruto in his Six Paths Sage Mode also has amazing abilities. However, if we consider raw power, Goku might be more powerful as he can reach levels of power that can destroy entire solar systems. But Naruto's intelligence and his ability to strategize could give him an edge in a fight.
In a combined fanfiction universe, it could be Whis from Dragon Ball Super. He is an angel who is extremely powerful and has knowledge far beyond most characters. He could potentially use his knowledge of the multiverse and his own powers to outwit and overpower anyone. Although Sailor Moon has the power of the Silver Crystal and Goku has his Saiyan strength, Whis' ability to manipulate time and space, as well as his overall power level, might make him the most powerful character. However, it also depends on how the author of the fanfiction wants to balance the powers of the two universes.
It's really hard to say. Naruto has some extremely powerful characters like Naruto himself with his Nine - Tails chakra and Sage Mode. But in Yu Yu Hakusho, characters like Yusuke at his Mazoku form are also incredibly strong. It might depend on how the crossover is written. If the power - up systems are combined in a certain way, it could be a character who manages to blend both types of powers most effectively.
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.
Rikuo Nura from Nura Rise of the Yokai Clan has a strong claim. He is the leader of the Nura clan and has both yokai and human forms. His yokai powers are vast, and if he were to gain some ninja skills from the Naruto universe, he could be nearly unstoppable. He has the ability to command a large number of yokai, which is a great asset in any battle.