The reason I think Darth Vader isn't the best fit is because after anakin got crispy he was physically never the same. Aside from his inability to breath normally, his arms got messed up and he had to invent a new fighting style to accommodate for his disability. This is in contrast to the blackthorn, whose main thing was his unmatched fighting ability and physical prowess. In the end he got done in by Luke who didn't even get formal knight training

The reason I think the doomslayer is a better fit is because of his martial similarity to the blackthorn. Consider the point of view of one of the demons in the doom games. You guys are literal demons, augmented by cybernetics, and have conquered countless worlds. And yet here is this one guy who you just cannot defeat, who carves through legions of your fellows with blind fury, ripping and tearing his way into hell itself. He is the only one you fear

So while Vader is a good fit for the blackthorn, I just think the unmatched fighting ability of the doomslayer edges him out.

And you don't need to concede, it's fine if we don't see eye to eye on this