Khamer, the Jerkass king of Ancheim, finally getting the Humiliation Conga he deserves once Agnès returns from the Wind Temple. It gets better if you do the sidequests and find out he was behind the water pricing scheme all along, and you finally get to give him a well-deserved asskicking.
Khamer also gets to GIVE an awesome moment. In the final world, Braev sends out a call to all the asterisk holders that he wants a showdown with the vestal, Edea, and their friends in Eternia, and that the jobmasters need to get over there. You'd expect Khamer to be hesitant, as he has been all about progressing the betterment of Ancheim through manual labor throughout the entire game, and Profiteur makes his business there. Instead, Khamer makes it clear he fully supports Eternia and plans to get over there with Jackal, Khint, and Profituer in tow. That's not the awesome part. The awesome part is when Profituer makes it clear he has no intention of going, and Khamer has Khint read a royal draft: Khamer will freeze Profiteur's assets if he does not cooperate. While he may have been a Jerkass, his loyalty to Eternia's cause was genuine, and he was willing to go out of his way to make damn sure Profiteur had no choice but to go with them. Even better, when Braev sets up his gauntlet, Khamer is put in the Final Sentinels, with the Grand Marshal himself... and he's the heavy hitter of the group. And Profiteur is in the Vanguard with Jackal.
The Red Mage sidequest is one for Ringabel. When Edea doesn't scream for help as DeRosa takes her to his lair, Ringabel was the first to go off and rescue her. He also calls DeRosa out when he said women were only tools to be used for his gain. Made more powerful in that DeRosa is practically Ringabel's Evil Counterpart. He may be a Handsome Lech, but even he's disgusted by borderline-rapist DeRosa.
Ringabel: A man who would call a woman his tool is no man! My friends will suffer no further insults!
How you get your proper Global Airship. Alternis destroys the Power Crystal of your first one, preventing it from flying. And even if it could, it wouldn't have been able to get over the mountains surrounding Eternia anyway. So you need to find a new one. After a series of seemingly unrelated events, you eventually find out that the Grandship can fly. Yes, the giant floating city is, in fact, your new Global Airship. The music that plays when flying really hammers this home. What player didn't let out a Big "YES!" the first time they heard that?
Assuming you do Qada's sidequest before Kamiizumi's, Kamiizumi gets fed up with Qada's boasts about his superiority, and shuts him up.
Agnès helping save the others and Egil on the way to the Fire Temple. Leading to the use of her Catchphrase in an unexpectedly badass way.
And then in one of the final battles with Airy her catchphrase gets another epic reprise, which simultaneously makes her the only party member whose dialogue in that fight has a solid comeback against the monster:
Agnés: We first met on that precipice overlooking the Great Chasm. You had come forth from the chasm's depths. But you had only just slain us! OUR BLOOD HAD YET TO DRY!
Airy: Indeed. You have served me well. Now, die a bitter death, knowing you served your enemy!
One for the Shieldbearers: with the heroes' instruction based on the bard's teachings, they spontaneously erupt into what the music choice for the scene seems to suggest is an a capella cover of the Bravely Default theme to defeat Praline A La Mode's music! Granted, we don't hear anybody singing, just the theme song playing in the background, but considering that the shieldbearers clearly didn't have any instruments on hand, there's virtually nothing else that it could be.
One for the villains: Victoria and Victor. Up to this point, you've taken down a brutal Monk while a White Mage was healing him, a pyromaniac pyromancer, a well respected Knight of the Eternian forces (Who, by the way, wiped the floor with every knight in Caldisla. ALONE.) a skilled thief, a swordsman capable of enhancing his blades, a dragon, a king who can partially control time, a Merchant who has monopolized water, a demonic dog, a skilled hunter, a woman capable of summoning a large earthquake creating lobster god, one of the strongest warriors of Eternia, and a man with mind controlling cologne and incredible magic. You probably feel like you can take on the world. Then Victoria and Victor show up. They proceed to leave you due to an untimely stroke on Victoria's part...if you were able to survive up to four turns. And those four turns are indeed hard to survive, let alone dent their health bars. They are really the only Hopeless Boss Fight in the game, and they let you know right off the bat that the council of six are NOT to be trifled with.
Another for the villains; when you fight Braev, he turns out to not exactly be good at damaging you, but staying alive? He soaks up damage like a sponge, and even after you beat him...He gets up and simply says:
Braev: You shall not pass.
It gets better. When you down him a second time, he grunts a bit and then simply yells:
Another subtle one, for The Jackal. Usually, when you fight him, you fight in the base where the thieves hang out. In the fourth world however, when you go to fight Praline, the battle goes as it usually does... until a turn after you kill her bodyguards while her HP is below a certain threshold, where Jackal jumps in out of NOWHERE to fight by her side. Note that Asterisk barriers appear to be designed with the intentions that no one outside can get in, and no one inside can get out. That isn't the awesome part. The awesome part is how he proceeds to somehow get to Ancheim, which could literally be impossible if the Water Crystal isn't active. THEN, he fights you again. In the past, he's been one of the few who always gets killed or is at least brought to the brink. In the fourth world, though, not only does he survive fighting you twice, he ends up doing it even when Khamer dies in his first fight in world four. Rather impressive.
In the True Ending, when you fight Ouroboros, he begins destroying alternate Luxendarcs to replenish his power, and threatens to destroy more worlds if the protagonists continue to fight him. They're about to give into despair when each the Heroes of Light in each of the THOUSANDS of linked worlds fight back against Ouroboros' planet eating magic, telling the protagonists not to give up. Cue Agnes striking back against Ouroboros with all the power in her pendant, shattering his horn and Ouroboros losing his mind with rage.
Bonus points if you haven't made any friends along the way, since it means the four worlds shown to accompany you are the four AI-bots that've been with you since early on.
Eternian Central Command is one in gameplay terms during chapter 8. The Eternian asterisk holders join forces to give you their best. They band together in groups of four (three for the first team) so that their respective jobs mesh such that you'll have your ass handed to you if you don't bring your A-Game. And then, when you finally win, Braev simply congratulates you and challenges you to come again when you feel you need your resolve renewed.
In the True Ending, Ouroboros's consistent regeneration is posing a problem for the heroes and the player - and then Lord DeRosso shows up right the hell out of nowhere. He tells the heroes he's going to sacrifice himself to stop it, gives each of them the advice they need to get going, then takes a battle stance... and then stops himself to deliver a triple-duty Badass Boast/Deal with the Devil/Stable Time Loop.
DeRosso: Ah, yes, I nearly forgot. Accept me, and I shall grant life everlasting. Though it shall be filled with grief, thou shalt have all eternity to wreak vengeance upon thine enemies.
Killing Victoria for the first time can count as this, bonus points if Agnes herself dealt the killing blow. Killing the psycho-bitch that murdered your only friend For the Evulz is pretty satisfying.
During the FMVs at the beginning of the game, Edea's intro features her sparring with Swordmaster Kamiizumi. Kamiizumi blocks most of her blows with Ise-no-Kami still sheathed. The second he draws it, Edea is on the floor, sword cut in two, with Ise-no-Kami at her head. Katanas Are Just Better? Maybe. Badass Teacher? Definitely.
In the Golden Ending, Alternis Dim, implied to be our Ringabel, is somehow sent back in time to his own world to fight Larval Airy and stop her from killing Edea again.
Lester DeRosso's account of his battles with Sage Yulyana many years ago mentions terraforming the landscapes of Luxendarc and shaking the very world itself. While it isn't experienced, his story is vivid and it sounds like a truly epic struggle went down.