It's likely that Ben with the Omnitrix would be one of the most powerful. He has access to a wide variety of alien forms, each with their own unique abilities. For example, Heatblast could deal with cold - based threats in the Rosario Vampire world, and XLR8 could outrun any fast - moving vampires or enemies. However, some of the powerful vampires like Alucard in Rosario Vampire also have incredible strength, speed, and magical abilities. So it could be a close call between Ben in his most powerful alien forms and the top - tier vampires.
Well, it's a tough call. Ben has the advantage of the Omnitrix which gives him a vast array of powers. His alien form Alien X is said to be nearly omnipotent. But in the Rosario Vampire world, there are some very powerful entities. Inner Moka, for instance, has been shown to have great combat skills and raw power. If we consider all aspects, though, Ben might have the edge in terms of versatility. He can switch between different powers quickly to adapt to different situations. But the powerful vampires in Rosario Vampire are not to be underestimated, especially in their own territory where they can use the environment and their knowledge of the magical world to their advantage.
Well, a cool storyline might be that the villains from the Ben 10 universe somehow find their way to the Rosario Vampire world. And they start causing trouble by trying to take over the school or capture the vampires for their own evil purposes. Ben then has to team up with Tsukune and the other students to stop them. Or perhaps there could be a story where Ben accidentally gets turned into a vampire - like creature because of some magic in the Rosario Vampire world, and he has to figure out how to reverse it while still using his alien powers.
It could be Ben when he transforms into Alien X. Alien X has near - omnipotent powers. He can manipulate reality on a cosmic scale. In a crossover, he could potentially use those powers to control the Nanites or even restructure Rex's powers in some way. However, he's also very difficult to control, so it's a double - edged sword.
It's really hard to say. Danny has some incredibly powerful ghost abilities like intangibility, flight, and his ghostly wail. But Ben has access to so many different alien forms, each with their own unique powers. However, if we consider the fact that Danny has a connection to the Ghost Zone which gives him an almost unlimited source of power in a way, he might be the most powerful. But then again, Ben in his Alien X form has near - omnipotent power. So it really depends on how you define 'powerful' in the context of this crossover.
It could be Ben when he transforms into Alien X. Alien X has near - omnipotent powers, like being able to manipulate reality. In a crossover, he could use these powers to easily defeat most enemies. However, the Power Rangers also have their strong Megazord combinations. If they could find a way to counter Alien X's powers, they might stand a chance.
Definitely Naruto and Moka. Naruto's energetic and positive nature would contrast with Moka's more reserved and sometimes vampiric aloofness. Their conversations could be really fun to read. Naruto would probably be curious about Moka's fangs and her vampire powers, while Moka might be intrigued by Naruto's ninjutsu.
Hermione and Tsukune Aono could be another great pairing. Hermione's thirst for knowledge would match well with Tsukune's curiosity. Tsukune, being a human in a vampire school, has had to adapt and learn quickly, much like Hermione in the wizarding world. They could exchange their knowledge about their respective worlds, and together they might come up with unique solutions to problems that arise in the crossover story.
The interaction between Viewtiful Joe and Moka would be really interesting. Moka's vampire powers and Joe's 'viewtiful' abilities could create some really cool fight scenes. Also, their different personalities - Moka's shy - vampiric duality and Joe's confident heroism - would make for some great dialogue and character development moments.
Gray and Kurumu would be an interesting combination. Gray is the cool, ice - using mage, and Kurumu is the succubus with her charm powers. Kurumu might be initially attracted to Gray because of his looks, but Gray would be all business. Their relationship could develop from Kurumu's attempts at flirting to a real friendship as they face common enemies together in a crossover story.
I think Klaus from The Vampire Diaries would also be a very powerful character in a crossover. He's an original vampire - werewolf hybrid with incredible strength and a very cunning mind. But Buffy has the power of the slayer, which gives her enhanced physical abilities and a sort of sixth sense for the supernatural. If they were to face off, Buffy's slayer instincts might give her an edge, but Klaus's raw power and immortality would make it a tough fight.
It could be Rose Hathaway. She's a skilled dhampir in Vampire Academy, trained in combat and very tough. In a crossover, her skills combined with her no - nonsense attitude might make her more powerful than some of the Twilight characters who are more used to a relatively peaceful co - existence in Forks.