The end of Blood Reaver, when Vandred/The Exalted dies. Not only does he guide The Covenant of Blood in awesome void warfare against superior odds but at the end he relinquishes control of the body back to the Exalted; allowing the daemon to suffer as the Covenant explodes.
Malcharion's Reawakening scene. Or, pretty much any scene where Malcharion is active.
The slaves standing up to Ruven. To be more accurate, A Navigator, 2 Normal Humans, and a mutant stand up to a fully armoured psychic sorcerer who could easily kill them with very little effort. Part of this is because Talos would kill Ruven immediately if the slaves were harmed, but they all draw weapons on him first.
Uzas, the human slaves and thirty raptors taking down Demonically Possessed Marine Caleb, ending with Lucrophys snarling I'll eat your eyes. Coming from a Chaos Space Marine, let alone a Raptor, that is a genuine threat.
Vandred maneuvering the Covenant of Blood to enable him to evacuate the Night Lords from Crythe, and prevent the Warmaster from firing on his brother captain's vessel, by using his own orders is very, very awesome, especially considering how Vandred was behaving before hand.
Talos pulling a Ceiling Cling on a Blood Angel squad and simply slaughtering them. When the last one retreats, he simply laughs and remarks that he wasn't expecting that.
First Claw annihilating Third Claw who outnumber and outgun them, by simply charging forward and cutting them to pieces. Helps that one of their members is mister Ax-Crazy and another is a Master Swordsman.
Talos achieves a Crowning Moment Of Awesome at the same time he crosses the Moral Event Horizon: Spending weeks torturing Tsagualsa's Astropaths, he finally has Octavia kill them with her third eye, ensuring that it was an extremely painful death. What makes this amazing is that he had Octavia do it when they were sitting right on an Astropathic duct used to transmit psychic messages, ensuring that the Astropaths death-cries were carried along and received by Astropaths on multiple Imperial worlds, resulting in warp storms that could last either days or centuries, and in a few cases resulted in planets suffering daemon attacks. It's utterly horrifying that Talos could inflict that much pain on so many worlds but there is no denying that he really harmed the Imperium with this.
Xarl's duel against the Genesis Chapter Champion displays just how impressive he is, by killing the champion where his four brothers failed, taking some glancing strikes from a thunder hammer, and headbutting the champion into submission. Only after the Champion is dead, his wargear granted to Xarl as trophies, that the Night Lord dies from his crippling injuries. It should also be mentioned, he is implied to have found and killed at least a combat squad of marines on his own.
If you remember that the Night Lords are the baddies, then the description of how they get owned by Codex Astartes loyalists on Tsangualsa is particularly good. A CMOA for Aaron Dembski-Bowden, too, because most Ultramarine-players consider it one of the best depictions of how the Codex Astartes should be - a guide, a philosophy for total war, written by the finest military minds in the Galaxy - not an American football playbook, but The Art of War in space:
Let's give a little background here. When we say that they sent the Ultramarines, we're not referring simply to the one thousand battle-brothers. We're referring to the Ultramarines and their successors chapters from the Second Founding. All of Them. It's established that they have numerical superiority before the battle even begins. The book even acknowledges that the Ultramarines and their successor chapters don't need tactics, and can simply overwhelm them with sheer numbers. They still use tactics designed to instantly counter the Night Lords every traditional strategy because the Ultramarines are that disciplined. Talos admits that the Codex Astartes just beat the Night Lords in the most well-executed and straightforward siege he has seen since the Battle of Terra.
Curze: It was the only way. Sevatar: No? What other ways did you try? Curze: Sevatar... Sevatar: Answer me, father. What politics of peace did you teach? What scientific and social illumination did you bring to this society? In your quest for a human utopia, what other ways did you try beyond eating the flesh of stray dogs and skinning people alive? Curze: It. Was. The. Only. Way.
Captain Malcharion, every time the story focuses on his perspective it is like reading words distilled with pure awesomeness. Especially so in his appearance in Void Stalker, where he does one of the most, arguably, heroic acts in a 40k novel; especially so since he is supposed to be one of the "bad guys".
Malcharion: My chronicle already ends in glory. Captain Malcharion, reborn in unbreakable iron, slaying Raguel the suffer of the Ninth Legion for the second time, before at last passing into eternal slumber. That is a fine legend, is it not? Marlonah: Yes, Lord. Malacharion: Who would ruin their legend with one last, untold tale? Who would cast aside the slaughter of an imperial hero in favour of saving a single human from death in the infinite dark?