It's already clear that Alucard wins. And I'm not saying this because I like him or something, I'm just standing by the facts. Let's look at them more closely.
Sebastian uses best martial skills and stuns, have a perfect aim, can control fire and can't be killed by bullets or any other weapons and can apparently regenerate his body part that were cut off, but not instantly (anime only). Plus he can turn into shadow...-things. It is not quite clear if Sebastian is able to shape-shift or not. His only weakness supposedly is salt, (manga only) that so far has only caused him pain in stomach when Ciel tricked him drink it. Sebastian's true form doesn't seem to have anything that spectacular, mostly because we don't know about it that much. But when Sebastian fought against Ash he managed to distract him with his hideous appearance but when he finished him Sebastian only used the brutal power, nothing supernatural.
Alucard might not be that acrobatic like Sebastian, than leaves him helpless against Sebastian's fast movements and jumping skills. However Alucard also has a perfect aim, but it is a bit pointless, seeing how bullets can't kill Sebastian. But since Alucard is the strongest vampire ever, he can resist fire, which is why Sebastian's fire control is pointless against him. That leaves Alucard with his more supernatural powers, such as going threw walls, shape-shifting, teleportation telepathy, hypnosis, summoning of Familiars, intangibility, the power of omnipresence, meaning spontaneously appear and disappear whenever he wants and at his Zero Level call GIANT devoured soul armies to his side and use much stronger shadows that Sebastian has been using so far. The biggest advantage however is the fast regeneration. Even if both of these creatures would be cutting each other endlessly, Alucard would take the victory for he can heal himself a lot fasted and finish his opponent more easier. Also his supernatural powers, even when he is at his usual stage, would outmatch Sebastian's usual combat skills. SO Sebastian is clearly a just a dog-food for Alucard's Familiars. Heh, ironic.
So in conclusion I would say that the victory would go to Alucard, one hellish vampire!