From the castaways' perspectives, the moments when Kara and Barry's powers first manifest serves as this as they had no reason to believe they would ever have anything more than human limitations. Kara's manifestation also serves as a minor moment for Barry; with access to nothing more than his own teenage scientific knowledge, Barry is able to correctly deduce how Kryptonians might manifest powers on Earth and why those powers are limited.
Amanda Waller, also known as the Bitch or the Devil incarnate, blackmailing three world-class assassins to throw the basis for the Justice League, creating a prison for Fort Rozz's escapees or cleaning after the Undertaking. And she's a normal human woman, only relying on her Manipulative Bitch prowess. Don't mess with the Wall.
After many scenes spent shaking in front of Malcolm, the Queens finally decide to read him the riot act when he decides to meddle into Oliver's life, threatening to blow the whistle on the Undertaking if he only thinks about telling Oliver he's his blood father.
After a thorough trashing, Eobard Thawne pulls his master card and dangles the possibility of saving Nora in front of Barry, confident that the Flash will take the bait and create the Flashpoint event... only for Barry to refuse the deal. See, being trained by the League of Assassins helped Barry to understand that death is a natural process and moving on is not only okay, it's a necessity. Ultimately, the Flash's greatest victory was to mature beyond his grief over his mother's death.
The Undertaking being utterly thwarted when Supergirl locates and destroys both earthquakes engines, then Green Arrow publicly delivering Malcolm to the police before unveiling the Justice League's creation to the crowds, sending a message to the world thanks to Waller arranging for journalists being present.
The Justice League and human defenders of Earth get one for successfully repelling the Dominator's invasion.
Points to Zoom for being the first antagonist to utterly and completely defeat one of the main three with ease.
The League of Assassins and the Justice League end up teaming up in order to take down Talia's League of Shadows once and for all, winning thanks to J'onn managing to turn off the meta dampeners blocking Barry's speed. Once that's done, the battle is over in seconds.
Who is the person who finally manages to take down the fearsome Talia al Ghul? Is it her father, the legendary Ra's? Her sister Nyssa? One of the trio, who all are supreme badasses? Nope, it's none other than Tommy Merlyn, who manages to kill her with an attack she never saw coming, avenging his former love Isabel and stopping her from manipulating his life all in one move.
The Grand Finale of Arc IV, Zoom's invasion of Central City, full stop. It opens with Zoom successfully bringing his army of Earth-2 metas to Central City and forcing the Justice League to retreat through sheer numbers, breaking out all his captured minions, the prisoners of Iron Heights, and even capturing Amanda Waller. The League is forced to flee to the Fortress of Solitude and put together a plan, leading to a final battle where Barry forces Zoom into a one-on-one speed duel across the city while the rest of the League, other superheroes who show up to help (including such powerhouses as Green Lantern and Zatanna), Task Force X, and the military lays siege to Central City to defeat the villains within. It's an epic action scene from start to finish.
Barry's final defeat of Zoom. In the grudge match that has been building the entire arc, Barry has finally closed the speed gap enough for his skills to let him keep pace with Zoloman, but not enough to give him a decisive edge. When Zoom takes the battle to Earth-2, Barry uses his intelligence to outmaneuver his nemesis, luring him to Earth-2 STAR Labs and activating the building-wide meta dampener. Both men lose their speed, leaving serial killer Hunter Zoloman against the League of Assassin's The Lightning. The following beatdown is a thing of beauty.