I think it would be Ben. His ability to transform into various aliens brings a lot of possibilities. He can be a tough fighter like Rath or a strategic thinker like Grey Matter. In the crossover, seeing how he interacts with the Gems and Steven would be really fascinating.
Steven could be the most interesting. His growth throughout the Steven Universe series is remarkable. He has such a kind heart and unique powers. In a crossover, he'd be the one trying to bridge the gap between the two very different worlds of Ben 10 and his own. His relationship with the Gems would also play a big role in how he deals with Ben and his adventures.
The Gems, especially Garnet, could be the most interesting. Garnet is a fusion of two Gems and has the ability to see the future. In a crossover with Ben 10, she could predict the moves of Ben's enemies or the potential outcomes of their combined actions. Her stoic yet caring nature would make for an interesting dynamic when dealing with Ben's more impulsive and alien - based powers.
Steven himself would be very interesting. His kind - hearted nature and unique powers would stand out among the Avengers. He could bring a new perspective to the team.
I think Harry and Steven would be the most interesting. Harry has his own burdens of being the 'Chosen One' in the wizarding world, and Steven has to deal with his role as a half - Gem. They could understand each other's struggles and learn from each other's ways of handling difficult situations.
Enid from OK KO and Pearl from Steven Universe. Enid is a bit of a cool, laid - back character with her own brand of strength, and Pearl is a very refined and skilled gem. They could have a great dynamic, like Enid teaching Pearl some modern - day Earth things, and Pearl showing Enid some of her elegant combat moves.
Definitely Steven and Dipper. Steven is so open - hearted and has a positive outlook, while Dipper is more skeptical and curious in a different way. Their different personalities could lead to some really interesting exchanges as they try to understand each other's worlds.
I think Steven and Zim would be the most interesting pair. Steven's kind - hearted and positive nature contrasted with Zim's evil and self - centered personality would create a lot of comical and tense moments.
Sakura and Pearl could also have an interesting dynamic. Sakura is a strong - willed kunoichi who has worked hard to become a great healer and fighter. Pearl is elegant and highly skilled in combat. They could exchange their knowledge of combat techniques and healing methods. Pearl could teach Sakura some of the more refined Gem - style fighting moves, and Sakura could show Pearl how to use her medical skills in more practical and efficient ways during a fight.
Definitely Dipper and Pearl. Dipper is always eager to learn about the unknown and Pearl is full of knowledge about Gem history and technology. Their conversations could be really interesting as they share their insights on different mysteries.
It could be Ben himself. Imagine him using his alien forms in the Naruto universe. His Heatblast form could be really useful against ice - based jutsu, and Four Arms could take on some of the physically strong ninja. He would bring a whole new element of power and chaos to the Naruto world, and his reactions to the different cultures and powers in Naruto's world would be interesting to explore.
I think the interaction between Ruby and Ben would be really interesting. Ruby is very energetic and has a strong sense of justice. Ben, with his experience of using the Omnitrix to save the day, could share some of his stories with her. They could also have some friendly competitions regarding their combat skills, like who can take down a group of Grimm faster, with Ruby using her Crescent Rose and Ben using one of his alien forms.
I think Ben's Heatblast and Thor would be an interesting combination. Heatblast can generate intense heat and fire, while Thor has his lightning powers. They could create some really powerful elemental attacks together.