The return of Captain Severus in a massive Roaring Rampage of Revenge right through the ranks of the Black Legion. Subverted when it turns out that it's actually the Daemon from before wearing Severus' body as part of a complex Batman Gambit.
Karnak the Chaplain gets one with two simple words.
"BURN,HERETICS!"
Novels
Nightbringer
Uriel Ventris giving The Nightbringer a You Shall Not Pass! moment and threatening to detonate a melta bomb that would bring down the tomb on top of both of them, definitely killing Uriel and quite possibly killing the weakened Nightbringer. Uriel looked a god as old as the universe in the eye and dared it to try and take his men, and the god backed down.
Given that in-universe the Nightbringer inspired the fear of death and one of its forms is a personification of death, it's as if he looked Death in the face, and Death backed down!
Warriors of Ultramar
Ventris gets a dual moment of awesome and heartwarming when, after learning that Inquisitor Kryptmann and Chaplain Astador lied to him and virus-bombed a planet rather than give the 4th Company the chance to evacuate the billions of people trapped there, Ventris decks Astador and very nearly strangles Kryptmann.
Pasanius gets one in that scene when he grabs Astador from behind before the nearly feral Chaplain can try and strike back and holds back a raging Astartes with no effort at all.
Dead Sky, Black Sun
Uriel gets a bunch of Awesome moments, but one of the best is the Rousing Speech he gives to the Medrengard Renegades that inspires them to join him and Pasanius on their Death Oath quest. It cost most of them their lives, but they died like real Astartes.
After nearly dying in unspeakable ways several times, Uriel gives a memorial to a guardsman that died to save them, praising him as a true servant of the Emperor. Which is interrupted by the Slow Clap of the renegade Raven Guard marine that joined them, "congratulating" them on what had so far been a suicide mission. Uriel is quick to lash back with a few words of his own.
Uriel: "Silence! You are not a Space Marine anymore. You're not even a man. You're a coward and a traitor."
The Chapter's Due
The entire novel is a whole Moment of Awesome for the Ultramarines. Having suffered several years of being a Creator's Pet in terms of fluff, Graham McNeill proves just why the Ultramarines are held as the standard for Space Marines: they're actual human beings who treat their people with dignity and respect.
Marneus Calgar: Having previously lied to his own Chapter about killing M'kar, Calgar uses the Shard of Erebus recovered by Uriel Ventris to kill M'kar. He emerges as a humbler man from the battle.
You all heard Varro's words. The daemon is vulnerable, exposed, and we have one chance to end this. Right here and right now, the fate of Ultramar is in our hands. You are the best and bravest of your Chapter, and though we may die in these mountains, we will die in service to something greater than blood, something greater than land. We fight for what we know is right. I will lead you in that fight, and all I ask is that you fight like the heroes you are!
Varro Tigurius: At his absolute limits, Tigurius manages to wake up long enough to sever M'kar's link to the Warp core aboard the Indomitable, making him unable to raise anymore daemons.
How he did it is a CMOA as well: even with his psychic powers, he is physically too weak to stand, so he is carried along by a dozen of the civilian refugees of Talassar who had gathered in the citadel of Castra Tanagra. Even Calgar finds it a CMOA:
Maskia Vollant held Tigurius' staff, though it was almost too heavy for him to bear. Calgar had never been prouder of his people than at that moment.
When Calgar leads the remaining Ultramarines outside the fort for a last-ditch attack on the weakened M'kar, dozens more civilians grab whatever weapons they can scrounge and join the charge. In the valley waiting for them is a literal army of daemonic creatures, led by a being with the powers of a god. What can these "mortals" possibly hope to accomplish that the greatest Super Soldiers in the galaxy cannot? Not one of them even stops to ask the question.
Uriel Ventris gets a crowning moment of awesome with this line: "I name thee Maloq Kartho! Your true and mortal name!"
Where Uriel defeats M'kar by invoking its True Name and thus allowing Marneus Calgar to stab it with the 'Shard of Erebus'. Made more awesome by the fact that Wordof God states plot points in the book hearken to the darkest and most secret times of the Horus Heresy - the Shard of Erebus seems to be a part of the Anathame.
Pasanius gets one when he takes a melta blast to the chest. His armor is reduced to dripping ceramite, yet he survives unscathed.
