In context, though, it was also a bit of a tearjerker…
Nyx saving Twilight from Nexus trying to hang her. And, by extension, when Nexus inevitably gets the ever-loving shit beaten out of him for what he did to her.
The expected pummeling did not happen. What did happen was far more calm, rational, and awesome. She demolished the huge cloud of the original Nightmare Moon's essence that had corrupted Spell Nexus and then proceeded to free the others from the portions of it that they had been corrupted with.
Spell Nexus sealing the Corruption inside himself before it could spread of its own volition. That took serious guts from a pony that Neighs Like A Little Filly.
Nightmare Moon versus every monster in Everfree. Even factoring in Me's a Crowd, that was epic.
On that note, Fluttershy stepping in and using The Stare on the Lupus Minors to send them running.
From the same fight, Pinkie Pie rigs decoys of herself filled with knockout gas to take out some monsters. They fool Nightmare Moon as well.
Applejack uses her rodeo skills to slow down a Cerberus.
The final boss is a Lupus Major. Remember the Ursa Major from Boast Busters? This is the wolf version of that, and like the Lupus Minors it's actively hostile. Nightmare Moon sends it running with a single lightning blast, visible all the way from her castle. The Lupus Major gets it's own CMOA for almost killing Nightmare Moon in the process.
Twilight Sparkle vs Diamond Tiara's Father, maybe not a major CMOA, but still, it does belong on this list.
The speech Nyx delivers when submitting herself to the Royal Pony Sisters is pure badass. Even the universe agrees, because it's later revealed that she earned her cutie mark just afterward.
Nyx: As an Alicorn, I can bear the burdens no other pony can. I threw myself in the way of a Lupus Major to protect Twilight because, while the beast wounded me, it could have easily killed Twilight. The pain I endured saved not just one life but lives all over Ponyville. It's far better for me to be wounded if it means somepony, anypony, can continue to live and be happy. I can be bruised, battered and beaten but, as long as there is still breath in my lungs, I will continue to protect ponies. I will protect them, because what can kill them I can survive, because what hurts them is but a scratch for me, because it's the one thing I've been able to do right.