Pan defeating Typhon. To put this into perspective, Typhon had just Completelytaken down all the other Gods, trapped Ares in a vase and was using Zeus as a mattress. Little Pan comes and does his Panic Yell. Typhon is stopped and Pan brings Zeus back to life and saves everyone.
Hephaestus getting revenge on Hera by making her own throne into a trap that holds her completely immobile while he gives her "The Reason You Suck" Speech and offers to be her best and favorite son if she accepts him.
Athena coming out of Zeus's head.
Zeus saving his siblings and uniting them against Cronus.
Zeus releasing the monsters bound in Tartarus, thus proving that he was a better leader then Cronus ever hoped to be.
DIOMEDES KILLS EVERYONE. HE RAMPAGES UP AND DOWN THE BATTLEFIELD AND FUCKS UP ALL THE TROJANS’ PLANS.
TROJANS ARE STABBED IN THE FACE. TROJANS ARE STABBED IN THE ARM. TROJANS ARE STABBED IN THE NIPPLE. LIMBS ARE HACKED OFF. TONGUES ARE CUT OFF. THERE’S A LOT OF BEHEADING. SPEARS GO THROUGH THROATS AND STOMACHS AND NOSES AND NIPPLES (LOTS OF NIPPLES). BRAINS SPLATTER ALL OVER THE BACKS OF HELMETS. DIOMEDES IS A FUCKING KILLING MACHINE.
AND THEN HE STABS APHRODITE, BECAUSE HE’S FUCKING INSANE AND SHE JUST HAPPENS TO BE THERE.
AND THEN HE STABS ARES. AND ARES RUNS AWAY.
DIOMEDES IS JUST THAT FUCKING AWESOME.
He also charged at Apollo to spear him and Aeneas (whom Aphrodite had tried to protect before, earning her a sword to the arm), thrice, Apollo only keeping him away by blinding him each time with a powerful flare of light.
And if that isn't enough (he was admittedly helped by Athena against the immortals) he defeated over thirty well-known warriors of the time, and outmatched most of his fellow Greeks in various sporting events (They had to stop his fight against Ajax, for fear he might kill the giant man).
The horrific monster Typhon, last child of Gaia and father of many monsters, strode towards Olympus to kill the gods, destroying cities and throwing mountains with his bare hands along the way. He possessed a hundred dragon heads, shot fire from his eyes and was so large his head scraped the stars and his arms spanned the horizon. He first attacked Zeus and easily best him, removing the sinews from his legs, crippling him, until Hermes restored them and healed Zeus. Zeus prepared for their last showdown on Olympus, all of the other gods (minus Athena, who stayed alongside her father) having fled to Egypt in fear. They fought, Typhon wielding fire, Zeus his thunderbolts, creating earthquakes and tsunamis with their blows, until finally Zeus cast Typhon into Tartarus.
Zeus may often be a hedonistic dick in legends, but that story confirms more than any other why he is the ruler of the gods. When all others fled before Typhon, he stood still.
Qualifies as a CMOA for Typhon to, who may well be the Ur Example of a Hero Killer. When you make the entire Olympian pantheon run in fear and defeat Zeus you are a badass among badasses.
To put extra emphasis on Typhon's defeat, Zeus slammed Mount Etna on top of him. He literally knocked Typhon from this dimension into Tartarus.
Some minor points for Athena too; she was the only God along with Zeus who didn't flee at the sight of Typhon. Could also count as a Crowning Moment of Heartwarming as it shows how loyal she was to her father.
Hades stayed at his post, too, although he is described to have been shivering in fear when the shockwaves from Typhon's battle with Zeus could be felt all the way down to Tartarus. Then again, when you think about it, that he was just as scared as everyone else, but still didn't flee makes it all the more impressive.
By the bucket load for Odysseus. Some pretty epic ones are his chewing out of Agamemnon, covering and protecting Ajax from a wave of countless Trojans while the other man was recovering Achilles's body, and of course the creation of the Trojan Horse.
Theseus killing the Minotaur with his bare hands. Granted, the beast was asleep but still impressive.
It was asleep when he started.
Hercules solving his 12 Labors. To elaborate:
Killing a lion, a flock of man-eating birds, a herd of man-eating horses and the hydra.
Capturing Artemis' sacred stag, a giant boar, a sacred bull, a herd of cattle belonging to an Eldritch Abomination and Hades' pet Cerberus.
Cleaning a really dirty stable by diverting 2 rivers into it.
Stealing the Amazon Queen's girdle and some golden apples by tricking Atlas.
Orpheus descending to the underworld to rescue Eurydice. And he almost pulled it off too.
Psyche did pull it off. Put into perspective, only six people had managed to do so. The other five were Aeneas, Heracles, Orpheus, Theseus* along with Pirithous (who didn't make it out) and he required Heracles' assistance and still didn't come out unscathed [all half divine] and Odysseus [who had divine ancestry.] Psyche was the only woman, pregnant, and completely mortal at the time, to wander down into Hades, finish her quest [asking Persephone for a box of beauty], and make it back to the surface even after having spent some time suffering abuse at the hands of Aphrodite and completing 2 or 3 previous quests for her. Epic.
Perseus killing Medusa. And if that wasn't enough, on his way home, he saved Andromeda from being devoured by Poseidon's sea monster, without any rest. Other myth sources say that he used Medusa's head (that he just decapitated) to petrify said monster.
Also, the other use he gave to Medusa's head — petrifying the corrupt king that sent him for it with the intention of killing him off so he could take Perseus's beloved mother as his wife. And then he didn't take the throne - in a Crowning Moment of Heartwarming he gave it to his kind adoptive father, who also happened to be the evil king's brother who should have reigned from the start, but was banished away by his evil sibling.
Sisyphus, whose life and repeated cheating of death straddle the line between this and Crowning Moment of Evil.
For additional context; Lucretia was the daughter of a Roman Prefect, and wife of a Governor during the last years of the Roman Kingdom. While the monarchy was unpopular, after Lucretia requested retribution which included killing herself during the debate on what to do, it is easy to assume that this is the point at which "King" became a dirty word in Rome. The subsequent revolution resulted in the Roman Republic being established.