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.
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.
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.
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.
There's also the idea that a character like Luigi, who is often overlooked but has his own unique skills, could become very powerful when combined with Naruto's abilities. Imagine Luigi using the invisibility power - up along with Naruto's stealth skills. He could be a formidable opponent in this fanfiction crossover.
It could be Naruto himself. His ability to use massive amounts of chakra, create shadow clones, and use powerful jutsu like the Rasengan and the Sage Mode would make him a formidable opponent. He could adapt his ninja skills to the Street Fighter fighting style easily.
Sasuke could also be a very powerful character in this crossover. His Sharingan abilities combined with some of the dark magic from the Harry Potter world could make him a formidable opponent. But then again, Hermione with her quick thinking and ability to master new spells could also hold her own against him. It really depends on how the crossover is set up and what abilities are allowed to be combined.
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.
It could be Naruto himself. He has an enormous amount of Chakra and his ability to learn new techniques quickly. If he were to partner with a powerful Digimon like Omnimon, he would be nearly unstoppable. His ninja skills combined with Omnimon's power would make him a force to be reckoned with.
Probably a combination of Naruto and Hei. Naruto has his immense chakra reserves and powerful jutsu like the Rasengan and the Nine - Tails' power. Hei, on the other hand, has his Contractors' abilities which include superhuman strength and agility as well as his unique weapon skills. Combined, they would have a vast array of powers at their disposal and would be extremely difficult to defeat.