I think Alice could be a very powerful character in the crossover. She can see the future, which would give her group a huge advantage. They could avoid dangerous situations with walkers or other threats before they even happen. And her speed and fighting skills as a vampire would also make her a force to be reckoned with.
It would probably be Naruto. His ability to summon large amounts of chakra and perform powerful jutsus like the Rasengan and Sage Mode would make him extremely formidable against zombies. He could also use his chakra to sense the presence of the undead from a distance, giving him an edge in survival.
It's likely Klaus Mikaelson. He's an Original vampire - werewolf hybrid in The Originals. He has super strength, speed, and his werewolf side gives him an edge in combat. In comparison, the vampires in Twilight, while strong, don't have the added advantage of being part werewolf. Also, Klaus has a lot of experience and is very cunning, which would make him a formidable force in a crossover situation.
Most likely Alucard. In the Hellsing universe, he is an extremely powerful vampire with a vast array of abilities. In comparison, the vampires in Twilight, while having their own strengths, seem rather weak when pitted against Alucard. He can manipulate shadows, has superhuman strength far beyond what we see in Twilight vampires, and his immortality is of a different level altogether.
Naruto himself would be very interesting. His ability to see the good in people and his strong willpower would contrast greatly with the bleak and often hopeless world of The Walking Dead. He would bring a new perspective to the survivors there, and his ninja powers would make him a force to be reckoned with against the zombies.
It's likely that Master Chief could be considered one of the most powerful. His combat training, physical enhancements, and access to advanced UNSC technology give him an edge. He has experience fighting large - scale battles and powerful alien threats. However, Isaac Clarke should not be underestimated. His knowledge of engineering and ability to survive against the Necromorphs in the Dead Space universe is remarkable. In a crossover, his skills could be adapted to deal with Halo - related threats as well.
However, it could also be someone like Madara Uchiha if he were to be part of this crossover. His vast power, knowledge of jutsu, and his ability to control the Nine - Tailed Fox would make him a formidable force. He could easily decimate hordes of zombies and manipulate the situation to his advantage, even in the unfamiliar highschool setting.
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.
It would probably be one of the older and more powerful vampires from Twilight, like Carlisle or Aro. Carlisle has centuries of knowledge and self - control, which would be very useful in the Hunger Games arena. Aro, with his mind - reading ability, could easily outwit most of the other characters. He could anticipate his opponents' moves and manipulate situations to his advantage.
Naruto and Rick would be a really interesting combination. Naruto's optimism and never - give - up attitude could inspire Rick, who has seen so much horror. And Rick's leadership skills and survival instincts could teach Naruto a thing or two about dealing with a post - apocalyptic world. They could work together to lead their respective groups and fight off the walkers and any other threats.
Michonne and Sam Winchester would be a great pairing too. Michonne is a skilled fighter with her katana, and Sam is very intelligent and well - read. They could work together to research any possible links between the walkers and the supernatural. Sam could also help Michonne come to terms with any new, otherworldly threats they encounter.