Simple enough: who is the hardest character in all of 40K?
Now, the rules should guide your decision, but so should the fluff; we're not talking 'two characters face off in a white room''. We're talking 'two generals face off with opposing armies, stratagems, etc, and fight to the death'.
Taking both fluff and crunch as a whole: who is the hardest?
My gut says Calgar, because as the 'Marine of all Marines', he pretty much should be, but he just never comes across in the fluff as ruthless enough to me. He'd worry about the civilians instead of putting a bullet in the bad guy.
Abaddon talks a good game, and is a beast on the table, but as we all know: he just can't back it up. Twelve Black Crusades later and he there kind of reaches a point where you have to figure that while the lunatic army with the scavenged weapons and mostly broken technology is part of the reason, there's probably another, more significant one, and it wears a top knot.
Draigo is too Mary Sue to be credible as a hard man for me. Same for Ghazghkull; he's not as bad as Draigo, but only because he's so charmingly violent. Yarrick's not a bad call, but he's not Tough enough (in game terms). The Swarmlord's a credible champ, but Tyranids always lose in the fluff, which makes him hard to place in front.
So in sheer, bloody-minded, gets-the-job-done, hammer-to-kill-a-flea, 'I gave you magic in there' Charles Bronson hard mannery, I think I'm going to go for an outside bet and choose Chaplain Lemartes, because he's already a monomaniacal lunatic to begin with, but then he hulks out and enters BEAST MODE and becomes something else entirely. Maybe not the best general, sure, but definitely the man most likely to beat you to death with your own spine.
What do other people think?