The entire fic in general: it took the Angst? What Angst? ending of "A Canterlot Wedding", destroyed it, and made Twilight, her friends, and her family sucessfully make up for what happened at the rehearsal. And the resolution matched up sucessfully with the heartwarming tune of the show.
The fight with Garble's gang. Rainbow Dash leads Thrash into Ghastly Gorge and tricks him into causing an avalanche that buries him, Fluttershy uses the Stare on Mumble and leaves him sobbing on a boulder in time out, and Twilight gouges out Garble's eye before threatening to kill him if he ever threatens Spike again.
Both Clyde admitting how much he searched for Pinkie after she ran away, and despite that he figured out that Pinkie was messing with her own route by circling and doubling back he still couldn't find her because she didn't want to be found.
Celestia literally laughing in Olive Branch's face when he reveals his true motive for staging the conspiracy. When Olive Branch desperately tries to save face by revealing he has Twilight hostage, Celestia coyly asks him if he truly thought a "standard issue" magic suppressor would be enough to stop her personal student. Cut to Twilight effortlessly destroying her horn collar and curb stomping Olive Branch's minions after she discovers what they plan to do with Spike.
Olive Branch tries to use the fact that Speedy Delivery has Rainbow Dash & Soarin' as hostages to make Princess Celestia gives in. Not only does Celestia refuse citing that both ponies would, in their loyalty, never want Celestia to risk Equestria for just the two of them, when we next see them, Rainbow Dash and Soarin' are not only unsurprised, but pleased, that Celestia's staying strong.
Scootaloo finally standing up her dad after all the crap he's put her through, followed by Rainbow Dash kicking his ass.
The first person to face Rabble Rouser and his anti-Spike mob? Mrs. Cake.
"Maybe you didn't hear me," the mare said, as icily as possible. "Get. Away. From. My. Bakery."
Twilight subduing Rabble Rouser's entire mob simply by listing off the various offensive spells she knows ... well, the first 24. And then Applejack gets a moment by knocking him out with one kick.
More cowardly ponies would fall to the ground right now and wait for the Chief of Police to arrest them.The mob agrees. And obeys.
Celestia gets one when it's revealed that not only did she know about the secret passageway in the guard barracks that leads outside the palace grounds, but that she pretended not to know about it specifically for a case like Olive Branch's where he had co-conspirators within the Royal Guard itself.
Olive Branch's ultimate punishment, as delivered by Luna: he's forced to feel all of the pain his actions have either directly or indirectly caused. And he'll continue to feel any future pain he causes.
Twilight nearly capturing Sunset Shimmer when she steals the Element of Magic. All because when Twilight woke up and saw her, she didn't just see a thief, but a potential threat to Spike.
Rainbow Dash calling Applejack out on her Pride and stubbornness.
The fight with Trixie's gang:
Applejack holding her own against Red Hoof. Granted, she ends up in the hospital, but she's released three days later.
Rainbow Dash takes out Lightning Dust by first playing off her short temper to make her act rashly, then grabbing her and flying at near Sonic Rainboom speeds to knock her out from the g-forces. And then she saves Applejack by choking out Red Hoof with a cloth garrote.
Rarity proving herself an Action Girl, much to Trixie's shock.
Fluttershy helps Applejack and Rainbow Dash to get the upper hand in their fights — in the former, by temporarily blinding Red Hoof with a piece of cloth around the eyes, and in the latter, by biting Lightning Dust's wing, distracting them enough for her friends to win. And then she beats Trixie by strapping a device designed to drain the Alicorn Amulet's magic on her horn.
Rarity Shaming the Mob to prevent vigilante justice from being taken on Trixie. And this is after what Trixie did to Sweetie Belle.
A more cathartic moment (in-universe and out) than awesome, but after the fight with Trixie, Dr. Malpractice is fired and sent back to medical school, due to failing to notice the blatant signs that Sweetie was being affected by dark magic.
The fight with the changelings in the llama temple.
Before that, The Changeling Queen, thinking that the Mane 5 are still wracked with guilt for how badly she tricked them with her Cadence disguise during the Royal Wedding the previous year, rubs it in their faces to give herself a psychological advantage. Much to her shock, they respond by laughing their heads off and mocking Crysalis for actually thinking that was going to work, since they've accepted that their mistake has happened and moved on. Rarity even admits she wasn't expecting Crysalis to understand.
When Discord shows up, Pinkie Pie immediately figures out that it's the Changeling Queen in disguise based on stuble clues.
Through the Mirror
When Twilight and Spike arrive in the human world Twilight is able to pick up on how things work just by observing her surrounding. Whether it's seeing how someone drives a car, how a road works, or how to use silverware she just observes and learns. Celestia even picks up on this and lets her figure out how to do research on the Internet herself through trial and error after directing her to a secure encyclopedia.
Despite only being at CHS for one day, Twilight already has a few students promising to vote for her just for standing up to Sunset.