If we combined Itachi from Naruto with Yin from Darker than Black. Itachi is known for his powerful Sharingan abilities, such as genjutsu and the ability to see through attacks. Yin has her Contractors' powers which are quite strong in combat. Together, they would be a force to be reckoned with. Itachi's strategic mind combined with Yin's combat - oriented powers would make them a very powerful entity. They could use Itachi's genjutsu to mislead opponents and then use Yin's physical abilities to finish them off.
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.
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.
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.
I think Naruto would be the most interesting. His positive and determined personality would contrast greatly with the darker and more cynical world of Darker than Black. He would be constantly trying to find the good in people and situations, which could lead to some really interesting character interactions. Also, his powerful jutsu and unique abilities would make him a force to be reckoned with in the crossover.
If you combined Naruto's determination and power - up abilities with the Otsutsuki's unique dojutsu and chakra - related powers, and then added the Akatsuki's knowledge of various jutsu and their strategic thinking, it would be an extremely powerful character. Naruto already has the Nine - Tails' chakra and his Sage Mode. The Otsutsuki have things like the Rinnegan. And the Akatsuki members are no slouches in combat either. So this combined character would be able to take on just about any foe.
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.
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.
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.
I think it could be a combined form of a character. For example, if Issei were to somehow gain the power of the Nine - Tailed Fox. He already has his Boosted Gear and his experience in battles in the Highschool DxD world. Adding the Nine - Tailed Fox's chakra would make him an extremely formidable opponent. He could use the fox's chakra to enhance his Sacred Gear even further, and his new form might have abilities that are a combination of both universes, like using the fox's tails as a weapon while also having the power of the Boosted Gear.