Actually, it could be a combination of Ben and Rex. If they were to somehow merge their powers, either through some kind of accident or a new invention, they could create a being with the best of both worlds. This new entity would have Ben's alien forms and Rex's Nanite control, and could be extremely powerful. It would have the brute strength and unique abilities of Ben's aliens, along with Rex's ability to heal and create various tools and weapons with his Nanites.
Rex might be the most powerful in some situations. If his Nanites evolved to a point where they could assimilate the technology of the Omnitrix, he could gain all of Ben's alien powers and then some. His ability to rapidly adapt and create new constructs with his Nanites gives him a lot of flexibility in combat and problem - solving. Also, he has more experience dealing with technology - based threats compared to Ben.
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.
I think an evil scientist who has found a way to combine the power of the Omnitrix and the nanites in a wrong way could be really interesting. He could be creating hybrid monsters that are very difficult for Ben and Rex to defeat. He would be constantly one step ahead because he understands both technologies so well.
In a fanfiction crossover, they could end up in each other's universes by accident. Ben finds himself in a world full of EVOs and has to rely on Rex to get back home. Meanwhile, Rex is intrigued by the Omnitrix and tries to figure out how it works. They face many challenges together, like dealing with Rex's old enemies who see Ben as a new target.
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.
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.
At first, they could be rivals. Ben might think Rex's nanites are too unpredictable, and Rex could see Ben as a bit of a show - off with his Omnitrix. But as they face common enemies, they start to respect each other. For example, when they're both captured by a powerful villain, they see how the other fights and starts to understand their strengths.
Another plot could be that an interdimensional rift opens up, and it's causing chaos in both Ben's and Rex's worlds. They discover that the source of the rift is related to the technology of both the Omnitrix and the Nanites. Together, they journey through the rift to find the cause and close it. Along the way, they meet alternate versions of themselves and their friends, which adds an interesting twist to the story.
One idea could be that Ben and Rex have to team up to stop a new villain who has the ability to disrupt both Omnitrix - based powers and Rex's nanites. They start off not trusting each other but as they face challenges, they learn to work together.
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.
Definitely Ben as Four Arms and a Machamp. Four Arms is super strong already, and Machamp is known for its four powerful arms and great strength. Together, they could take on almost any physical threat with their combined brute force. Their punches and grapples would be unrivaled.