This is a tough one.
Angron- A killing machine built for war, one of the larger Primarchs he had a mountain land on him and got back up wanting a fight.
Russ- Cunning and known as a fighter, no he did not beat the Emperor in the fighting challenge and as for him being the only one to best Horus in a duel im sure LEJ and Fulgrim also did so, also Corax attacked the Warmaster.
My vote has to go for Angron, if he gets Gorechild and Gorefather in close to Russ he is gone, though Russ does have Gormenjarl that is said to have been able to cut entire mountains in two.
Nyctophobia- Fear of the Dark Angel.
"No one ever spoke about of those two absent brothers. Their separate tragedies had seemed like aberrations. Had they, in fact, been warnings that no one had heeded?"
'Killing a man is like fucking, boy, only instead of giving life you take it. You experience the ecstasy of penetration as your warhead enters the enemy's belly and the shaft follows. You see the whites of his eyes roll inside the sockets of his helmet. You feel his knees give way beneath him and the weight of his faltering flesh draw down the point of your spear. Are you picturing this?'
'Is your dick hard yet?'
''What? You've got your spear in a man's guts and your dog isn't stiff? What are you, a woman?'