In a fight between characters from Ben 10 and Star Wars, it's a complex matter. Ben's Heatblast form could deal a lot of damage with his fire - based attacks. But a Sith Lord like Darth Maul, with his double - bladed lightsaber and acrobatic fighting style, would be a tough match. If we look at the most powerful in terms of long - range attacks, Ben as Cannonbolt could roll into a ball and cause destruction from a distance. But in Star Wars, a character like Han Solo with his blaster skills could also hold his own. Ultimately, it's difficult to determine who would be the most powerful as both universes have their own unique power systems and characters with diverse abilities.
It's hard to say for sure. Ben with the Omnitrix has a wide range of powerful alien forms. But in the Star Wars universe, characters like Yoda with his mastery of the Force are extremely powerful. If Ben transformed into Alien X, which has near - omnipotent powers, he could be really tough to beat. However, someone like Darth Sidious with his dark side powers and political manipulation skills could also be a formidable opponent.
It could be Naruto himself. His ability to tap into the Nine - Tails' chakra gives him an incredible power boost. In the merged universe, he might be able to use the Force as well, which would make him an unstoppable force. With his speed, strength, and now potentially the power of the Force, he could easily defeat most opponents.
It's hard to say for sure, but Master Chief could be a strong contender. His combat training and advanced armor give him an edge. However, Darth Sidious is extremely powerful with his dark side abilities. He could potentially manipulate Master Chief if he got the chance. But then again, the Forerunners from Halo have such advanced technology that they might be able to counter the Force -powers. So it really depends on how the story is written.
However, some might argue that Levi Ackerman could hold his own. His incredible speed and combat proficiency with blades could potentially make him a match for some of the Star Wars characters. If he were to get his hands on a lightsaber, he might become an even greater threat. But in the grand scheme of things, when facing the more powerful Force - using characters, he might still be at a disadvantage.
Definitely Ben Tennyson himself. His ability to transform into various aliens would add a whole new dynamic to the Star Wars universe. Also, characters like Darth Vader would be great. Imagine the confrontation between Ben in one of his powerful alien forms and Vader with his lightsaber and the Force. Another interesting character could be Yoda. Yoda's wisdom and his unique view of the Force could interact in an interesting way with Ben's more action - based alien powers.
In Vampire Knight, Yuki also has some unique powers especially as she is a special kind of vampire. In Harry Potter, Harry himself has the power of love and the ability to use powerful spells like Expelliarmus and the Patronus Charm. If they were to fight, Yuki's vampire instincts and her connection to the vampire world might make her a formidable opponent. But Harry's magical training and his courage could also tip the scales in his favor. It would be a very interesting battle to imagine.
Probably Great Red from Highschool DxD. He's already an extremely powerful being in his own universe. If he were to encounter the power of the Omnitrix and all the alien forms in Ben 10, he could potentially assimilate or use those powers in some way to become even more powerful.
Kakashi Hatake could also be in the running. His Sharingan and vast array of ninja techniques make him a formidable opponent. In the Star Wars side, Ezra Bridger has a strong connection to the Force. But Kakashi has more combat experience and a more diverse set of skills. If they were to face off, it would be an interesting battle. And of course, Naruto's ability to inspire and bring out the best in others could also be considered a form of power, which could tip the scales in his favor in this colliding universes scenario.
Obviously, Ben himself is one of the most important characters. His growth and awakening are at the core of the story. Rey could also be very important as she might play a role in his awakening, either as a friend, a mentor, or just someone whose actions inspire him.
Luke Skywalker is also a very popular choice. He could be a mentor - like figure to Ben, teaching him about the Force while Ben shows Luke his alien forms and the powers they possess. Yoda could also be a great crossover character. His wisdom and connection to the Force could be really interesting when combined with Ben's more impulsive and action - based nature from the Ben 10 series.
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.