Another strong combination could be Gwen from Ben 10 (since she often teams up with Ben in the series) and Scarlet Witch. Gwen has her magic powers, and Scarlet Witch has her reality - warping abilities. Together, they could enhance each other's magic and create illusions, manipulate minds, and even change the fabric of reality on a large scale to defeat their enemies.
A very powerful combination could be Ben as Alien X paired with Doctor Strange. Alien X has near - omnipotent powers, and when combined with Doctor Strange's magic, they could be an unstoppable force. They could manipulate time, space, and reality in ways that no single villain could counter.
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.
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.
I think Spider - Man from the Avengers and Four Arms from Ben 10 would make an interesting combination. They both have enhanced strength and agility. They could have some really cool fight scenes together, and also some humorous exchanges. Four Arms' brute strength combined with Spider - Man's acrobatics would be a sight to see.
Gwen Tennyson is often paired with Ben 10000. Since they are related in the original series, it's interesting to see how their relationship might be different in a crossover scenario. Maybe they have to work together to save the universe in a more complex way than before.
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.
A really cool character combination would be Hawk Moth and Vilgax. Hawk Moth's ability to akumatize people and Vilgax's thirst for power and his vast array of weapons and skills would be a terrifying combination. They could team up to take over both the Miraculous and Ben 10 universes. On the other hand, Tikki and Heatblast could also be an interesting pair. Tikki's positive energy and Heatblast's ability to generate and control fire could lead to some unique interactions. Maybe Tikki could help Heatblast control his power better or they could use their combined powers to defeat a powerful enemy.
I think Omnimon and Thor would be a very powerful combination. Omnimon has incredible strength and a wide range of attacks, and when combined with Thor's godly powers and his mighty hammer, they would be almost unstoppable. They could take on the biggest threats both in the Digital World and in the Marvel universe.
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.
Definitely Tony Stark and Tyrion Lannister. They are both intelligent, witty, and a bit of an outcast in their own worlds. Fans love to imagine them coming together, sharing their knowledge and maybe even coming up with some grand plans to either take over Westeros or save the world from some new threat.
I think it could also be a character like Itachi. His Sharingan gives him incredible combat abilities such as genjutsu and enhanced perception. If he were to get his hands on Halo technology, he could use it in very strategic ways. For instance, using a cloaking device to enhance his already stealthy nature from his ninja skills. And his intelligence would make him a force to be reckoned with in any 'naruto halo fanfiction' scenario.