Ok, I know I am way late for this discussion, but I had to add my sauce to this. I just feel like Alucard isn't really understood.
So first of all, I don't want to claim, that I know who would win, because all I know about D, is from the Vampire Hunter D Fandom. My impression is that he is indeed, very strong, very fast and extremly hard to kill. So I simply don't know.
However, D might be able to stop regeneration, which could be a problem for Alucard, because there is definitely no way he's winning without it. Yet, I would say that Ds ability wouldn't work in this case, because Alucard isn't regenerating per se. He doesn't have a body in the classical sense. Anytime he gets shredded, it becomes obvious that he doesn't really have a body made of flesh and blood. It's more like raw power, his and all the souls he has devoured condensed into a mist. That in turn makes it hard to really hurt him. Most weapons don't even hurt him, which is shown in the first episode when the lower Vampire unloads a smg onto him and he isn't wounded one bit. So, armor piercing wooden needles, however powerful, wouldn't necessarily hurt him, or more accurately, kill one of his souls. In order to kill a soul, he needs a weapon that is "magical". Ds sword is probably enough to do the job, and as far as I can tell, he can do some neat things with that. Now, lets not forget, that Alucard himself is a very good swordsman, although Ds superior strength and speed probably would overpower him.
Alucard has do a possibility to defeat D, even if only for a while, since i gathered that he can resurrect himself (which seems to me that makes him quite literally immortal, but as I said, don't know the Dverse enough). Alucard has not only his powers, but also the powers of his opponents, which would allow Alucard to surprise D and turn the tides.Not to mention, being able to switch from close to far range very quickly as seen in the battle against Anderson.
Another point for Alucard is, that as mentioned befor, he doesn't have a body, which means he can't get tired, at all. He has no stamina to loose. D on the other had, has a body and although I am sure that D can fight for an enormous amount of time, he has to kill Alucard 3 million times. Even if D is much stronger than Alucard, he is not strong enough to instantly kill Alucard, the moment he regenerates. And even if he killed a soul every second, it would take him nearly 5 weeks to truly kill Alucard.
I would also like to state that powers such as hypnosis, telekinesis don't play a roll, since both have them, and although one might have an advantage, I doubt it will make a big difference, since either one will be able to break free.
That being said, these kind of match-ups are quite difficult, because a lot comes down to the rules of the particular verses and how certain abilities would work if the two world would become one.
Oh, and little thing, the Jackal is very powerful, explicitly made to hunt vampire and other "freaks", so I would assume,that if D took a hit from one, it would **** him up a bit. I mean, it nearly stopped Andersons healing capabilities, which yes, are lower than Ds but still. There is again the question of how do the rules work