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.
It could be Ben when he transforms into Alien X. Alien X has near - omnipotent powers, like being able to manipulate reality. In a crossover, he could use these powers to easily defeat most enemies. However, the Power Rangers also have their strong Megazord combinations. If they could find a way to counter Alien X's powers, they might stand a chance.
However, one could also argue that Alien X from Ben 10 Omniverse, if combined with Naruto's willpower and determination, would be the most powerful. Alien X has reality - warping abilities, and Naruto's strong spirit could potentially control and direct those powers in ways that we can't even imagine in the context of the fanfiction.
The Red Ranger would likely be a top contender for the most powerful. They usually have strong leadership skills and often the most powerful attacks among the team. However, if we consider villains, Rita Repulsa could be very powerful in an Infinity War scenario. With her magic and cunning, she could cause a lot of trouble for the Rangers.
Another possibility is Doctor Strange. His magic is already very powerful. If he were to gain some of the unique abilities from Ben's aliens, like the XLR8's super speed for his spell - casting, it would make him even more formidable. The power balance really depends on how the fanfiction writer chooses to blend the two universes' powers.
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.
Ben Tennyson and Tommy Oliver would be a great combination. Ben with his Omnitrix and Tommy, the experienced Power Ranger, could share their knowledge and skills. Tommy could teach Ben about leadership and teamwork in the context of the Power Rangers, while Ben could show Tommy the different alien abilities from the Omnitrix. They could also combine their powers in creative ways to defeat enemies.
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.
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.