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.
However, one could argue that the Volturi, as a collective in Twilight, could pose a significant threat. They have a large number of vampires under their command and a certain level of organization. But still, Alucard has shown the ability to take on entire armies by himself. So even against the Volturi, he would likely have the upper hand. Plus, he doesn't play by the same 'rules' as the Twilight vampires, which gives him an edge.
It's also possible that Integra from Hellsing could be a very powerful force in this crossover. She has the authority to command Alucard and the rest of the Hellsing organization. Her strategic mind and leadership could be the key to dealing with the Twilight vampires. She could manipulate situations to her advantage and use Alucard's power in ways that the Twilight characters might not expect.
If you want the clearest chapter reference, go to Chapter 95. Early on, Hearing others praise her daughter, Aunt Lin was smiling so wide she...; later, the chapter leans on Lingling really is a good child..
If you want the clearest chapter reference, go to Chapter 70. Early on, So, under the curious gazes of everyone, Jiang Yan strutted up with his...; later, the chapter leans on He didn't even take off his sunglasses before he started reading into the....
Chapter 84 is the strongest chapter-level answer because it starts from When the villagers tasted his curry beef one by one, their faces revealed... and follows through on Why am I tasting sand in my mouth?" Wen Jiaojiao's bravado gradually....
If you want the clearest chapter reference, go to Chapter 120. Early on, Facing the sudden and somewhat strange request from Jiang Li, Director Sun...; later, the chapter leans on But, as if by some mysterious force, he still instructed his assistant to....
A practical answer is Chapter 127, because it turns the idea into a concrete scene. The chapter opens with Witnessing Jiang Li leave with a bucket, Jiang Yan refused to admit..., adds pressure through [LOL, I'm dying laughing, the Young Master is cursing heaven and earth..., and ends on He Yun.
Look to Chapter 147 for the clearest answer. It begins with Xiao Xueyi nodded in agreement after listening., keeps building through Su Yinwan seemed to have matured overnight, becoming more and more adept..., and lands on Only Jiang Li glanced back at Director Sun, and then took out her own....
A practical answer is Chapter 105, because it turns the idea into a concrete scene. The chapter opens with But this did not elicit any sympathy from the woman on the phone;
A practical answer is Chapter 84, because it turns the idea into a concrete scene. The chapter opens with When the villagers tasted his curry beef one by one, their faces revealed..., adds pressure through [Pfft, and she's teaching others how to cook, isn't it just scrambled eggs..., and ends on.
Chapter 71 is the strongest chapter-level answer because it starts from Shu Yan curled her lips with a hint of jealousy, "When can it be my turn... and follows through on Now they had to work on an empty stomach..
If you want the clearest chapter reference, go to Chapter 129. Early on, When they saw that the live chat was full of praise for Jiang Li, those...; later, the chapter leans on Had they been used as pawns by publicity accounts?!.