Well, lets look at this logically. Sebby is pretty much out of the fight from the begging, he really is powerful and all, but he's a little out of his league here. So the real fight would be between Alucard and Dante, who a practically twins. I could see them getting in a fight over who stole who's outfit. But lets face it, in the end Dante has nothing that could possibly even begin to counter Alucard's OP regeneration. When it comes to Alucard, there are very few or none at all that can really top him, with the exception of omnipresent beings. Now that I think of it, Alucard is pretty much the most powerful non-omnipresent being I can think of. The only people I could think of that could beat him would be high-tier reality warps like Miku of the Vocaloids, Reimu of Touhou or Haruhi Suzumiya (If she was aware of her powers) who xould just simply think him out of existence.