Alucard the partly insane and battle loving vampire of the Hellsing organization who near singal-handedly brought the Iscariot organization, the church's elite group of monster hunters and Millenium, the group of deranged and insane Nazi vampire's led by the even more insane and war-loving Major, by simply releasing all of the souls he had eaten over his many years, their broken wills his to control, making the a literal flood of blood and bodies.
Who do I think would win?
The answer may suprise some of you.
James Heller, after a battle that would probably reshape most of the landscape...for several miles, would win.
Before you get all pissy, here are my reasons:
Physical Strength - Physical strength would be almost dead even, maybe a bit in Alucard's favor, if it wasn't for the hammerfist. While it's main purpose is to create shockwaves through the ground, it has been shown that James can use them to punch, like when he's consuming Orian Soldiers.
Speed - Well this should go without saying. While Alucard is much faster than humans, Heller can outrun cars, tanks and choppers. James takes this area.
Weapons - While Alucard does have those freakishly overpowered pistols of his James is, after all his mutations and upgrades are said and done, bulletproof and gets missles, tank shells, and chopper rounds shot at him every twenty minutes.
Defense - Alucard just freaks people out with his regeneration. James on the other hand has several mutation that make him harder to hurt, not to mention he can avoid being hurt pretty well on top of his huge sheilds that deflect almost any kind of damage, the only thing that ever could get through them was a hammerfist.
Regeneration - Alucard wins here easily. While both men can regenerate from almost nothing, Alucard can do it quickly and more likely than not a whole lot more.
Battle experiance - This is actually pretty even if you think about it. While Alucard was a military man before death and continued fighting for a very long time, he was still sealed for bits and pieces of time throughout history. James was in the army and consumed thousands of military and BLACKWATCH agents, not to mention however many military members the evolved, Mercer's forces, that he consumed. They all consumbed military as well and he got everything they had, that includes the DNA they already consumed.
Personal drives - Well for James the drive was initially revenge but after he found out his daughter was alive it changed to finding her and keeping her safe, even going so far as to be willing to rip New York to pieces just to find her. Alucard just fights for the sake of fighting and, while he has been shown to care for Seras and Integra, it's also been shown that he doesn't really respect their ways of thinking, whether it be Seras with not being ready to give up her humanity or Integra when he asked her if she felt good telling him to kill ignorent policemen.
Greatest abilities - Devastator vs soul army, I would say that this would be a tie. Devastator, after James consumed Mercer could cover the entire RED ZONE, a third of New York. While some of you may thing that the Soul Army was better, you gotta remember that James had to make sure to not destroy buildings, while Alucard just sent his army through a lot of them. If he didn't have to worry about buildings then the Devastator could cover similar range to the Army.
Absorbing - James wins here because he can do it quicker and easier. Also, since the souls inside Alucard are already mentally broken and pretty much made up of blood James could absorb them if they so much as touched him.
Also keep in mind one more thing, Heller's weapons also upgraded throughout the game. Consuming the origionater of the weapons made his devistator dozens of times stronger. Same would have happened for every other ability. Only reason that didn't happen every other time james consumed something that gave him a boost was because they only had one power. Mercer had better versions of all their powers so everything was upgraded vastly afterwords.
So there you have it. James would win. And before you get all defensive and say that I don't like Alucard or I like James more than him or some bullshit like that, I love both the Prototype game series and Hellsing and Hellsing ultimate.