I think it really depends on the context of the fanfiction. If it's set in the world of Overlord, Ainz might have an advantage because he knows the terrain and has his support system there. But if it's in Ben's world, Ben would be more familiar with the surroundings. Also, some of Ben's alien forms like Clockwork could manipulate time, which could be a huge advantage against Ainz. However, Ainz has his own time - related magic in some ways. So it's a very complex question with no easy answer.
It's hard to say. Ainz Ooal Gown has a vast array of powerful magic spells at his disposal. However, Ben when he transforms into Alien X could be extremely powerful as well. Alien X has reality - warping abilities. But Ainz has his intelligence and strategic thinking. So it could really go either way depending on the situation.
It could be really cool. Picture the powerful beings from Overlord in the world of Ben 10. Maybe Ainz Ooal Gown could use his magic against some of Ben's alien forms. The different power systems would create some interesting battles and interactions. For example, how would Ainz deal with an opponent like Four Arms? And how would Ben react to the undead hordes that Ainz commands? There are so many possibilities for epic crossovers in this fanfiction concept.
Ainz Ooal Gown would be a very interesting character to focus on. He has a complex personality and his vast power makes him stand out. In a crossover, seeing how he adapts to a different anime world's rules and cultures would be fascinating. He could be the catalyst for many interesting plot developments.
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.
It could be Ben himself. Imagine him using his alien forms in the Naruto universe. His Heatblast form could be really useful against ice - based jutsu, and Four Arms could take on some of the physically strong ninja. He would bring a whole new element of power and chaos to the Naruto world, and his reactions to the different cultures and powers in Naruto's world would be interesting to explore.
Ruby would be really interesting. Her energetic and optimistic nature would contrast greatly with the often dark and power - hungry characters in the Overlord world. She could inspire some of the downtrodden minions with her belief in the good of people. Also, her scythe skills and speed would be a unique addition to the battles in the Overlord universe.
In a crossover, it really depends on how the powers are combined. If Naruto could somehow combine the Nine - Tails' chakra with Sage Mode and then learn some ki techniques from the Dragonball Z side, he could be extremely powerful. But Goku has been training for a long time and has access to godly ki. So he would be a top contender as well. Vegeta too, with his own determination and power - ups, could also be in the running for the strongest.
It's hard to say. Goku in his Super Saiyan forms has incredible power and can destroy planets. But Naruto has the Nine - Tails' power and Sage Mode. If we consider Naruto at his peak with all the tailed beasts' power combined, he could give Goku a tough fight. However, Goku's ability to constantly increase his power level might give him an edge in the long run.
It's hard to say for sure. Naruto in his Sage of Six Paths mode is extremely powerful, with access to a vast amount of chakra and the ability to create and manipulate Truth - Seeking Balls. But Ichigo, when he reaches his final form, can also be a force to be reckoned with, having the combined powers of his Hollow, Quincy, Soul Reaper, and Fullbringer heritage.
I think the interaction between Ruby and Ben would be really interesting. Ruby is very energetic and has a strong sense of justice. Ben, with his experience of using the Omnitrix to save the day, could share some of his stories with her. They could also have some friendly competitions regarding their combat skills, like who can take down a group of Grimm faster, with Ruby using her Crescent Rose and Ben using one of his alien forms.
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.