The Valhallans' Last Stand against the Oblivionites.
The Grey Knights on Titan as well.
For clarification: the Grey Knights and Adeptus Custodes remnants (Super Prototype space marine bodyguards for the Emperor) have been fighting both the Necrons above Titan and Daemons below it for over 10,000 years. Just when they had been reduced to two rooms and lost their last two dreadnoughts, they hear the sound of bolter fire when the ammunition stores of the planet had long since dried up millennia ago. As it turns out, the Legion of the Damned has arrived and several of the Grey Knights' most holy relics come to life revealing a pair of Creepy Twins and a webway portal, allowing them to escape.
Roboute Guilliman BANISHING DRAIGO with the Gauntlet of Ultramar.
“Who and what I am would be exceedingly complicated to explain. But judging from your expressions, you may call me Revelation."
The Phoenix Lords' exploits:
Fuegam destroys a Necron warship by unleashing the sun that powers it.
Arhra and Karandaras' fight, which takes place while the reborn Khaine rises.
It should be noted that their duel takes place on top of Khaine, as the reborn god himself duels an Imperator Titan.
You thought the Pheonix Lords by themselves were awesome? In chapter 45 you get to see 8 of them on the same battlefield at once.
Abbadon forever escaping accusations of being General Failure with his victory in battle of Qualtha - planning for a Mêlée à Trois which weakens his enemy, destroying vast armada of Kor Phareon and smucking him down verbaly in a following exchange:
“You will not kill me,” Kor Phaeron explained, with the certainty of the eternal fanatic. “I cannot die. You shall see. The Gods shall pluck me from danger. I am too valuable. But if this ship is to die, it is fitting it takes you to the warp with it!”
“What makes you think you shall live?” Abaddon replied coldly to his enemy’s words.
“Faith,” purred Kor Phaeron ecstatically.
Abaddon had no fear. “Faith must always... be tested.”
This is especially awesome when you consider that the two chaos warlords are playing a game of 'Chicken' with two monumental, miles long battleships. Of course, neither of them backs down.
Vulkan's battle with Fulgrim
Iacob, the Last Good man in the Galaxy, walked on the skin of the Mad God, aka the OUTCAST C'TAN, and through his pure idealism, ACCEPTED it/her for what she is, and CONVINCED her to harbour the Refugee fleet and become a safe haven for refuge all over the universe. Note that this happened when so many crew members are becoming crazy/insane from being near the Outcast C'Tan. Truly, idealism and heroism all is not lost.