It could be Ben when he transforms into Alien X. Alien X has near - omnipotent powers. He can manipulate reality on a cosmic scale. In a crossover, he could potentially use those powers to control the Nanites or even restructure Rex's powers in some way. However, he's also very difficult to control, so it's a double - edged sword.
It's really hard to say. Danny has some incredibly powerful ghost abilities like intangibility, flight, and his ghostly wail. But Ben has access to so many different alien forms, each with their own unique powers. However, if we consider the fact that Danny has a connection to the Ghost Zone which gives him an almost unlimited source of power in a way, he might be the most powerful. But then again, Ben in his Alien X form has near - omnipotent power. So it really depends on how you define 'powerful' in the context of this crossover.
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.
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.
Probably Great Red from Highschool DxD. He's already an extremely powerful being in his own universe. If he were to encounter the power of the Omnitrix and all the alien forms in Ben 10, he could potentially assimilate or use those powers in some way to become even more powerful.
It's likely that Ben with the Omnitrix would be one of the most powerful. He has access to a wide variety of alien forms, each with their own unique abilities. For example, Heatblast could deal with cold - based threats in the Rosario Vampire world, and XLR8 could outrun any fast - moving vampires or enemies. However, some of the powerful vampires like Alucard in Rosario Vampire also have incredible strength, speed, and magical abilities. So it could be a close call between Ben in his most powerful alien forms and the top - tier vampires.
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.
Well, Danny Phantom has the ability to phase through objects and fly at high speeds, which is really cool. Ben 10 can transform into so many different aliens, each with their own unique powers. And Kim Possible is super athletic and has all those useful gadgets. However, I think Ben 10 might be the most powerful in a crossover. His Omnitrix gives him access to a wide variety of powers that can be used in different situations. For example, if they were in a fight underwater, he could transform into an alien that is adapted to aquatic environments. But again, it's all about how the story is written and what challenges they face.
In my opinion, it might be Doctor Manhattan from 'Watchmen'. He can manipulate matter at a sub - atomic level, is essentially immortal, and can see the past, present, and future all at once. His powers are so vast that he is almost god - like in the context of the story.