Hell even before Terra's final boss, he screams awesome. While Master Xehanort is telling him to give into the darkness, Terra curls in, and lets out an enormous roar that just screams 'I'm pissed as hell and not gonna take it anymore.' Shivers up the spine, man...
When Master Xehanort commits Grand Theft Me on Terra and emerges as the young Xehanort, he begins to walk away when Terra's armor reforms and conjures a barrier of light around the arena. First, Theme Music Power-Up because the battle theme is "Rage Awakened", the theme when fighting the Lingering Will. Second, Terra uses the Power Of Hate to become Animated Armor and battle Xehanort in his own body. Third, you get to take control That One Boss to deal a beatdown on the main antagonist of the series who up until now has been The Unfought. Fourth, for once the barrier around the arena isn't the boss's, it's Terra's — he's making sure Xehanort has nowhere to run. Fifth and finally, it's Xehanort. Not Ansem, not Xemnas, not Master Xehanort. After dealing with his alter-egos for five games before, you finally face Xehanort himself, and he's a Lightning BruiserMagic Knight with access to every single attack you've come to rely on during the game.
Ventus uses Heroic Willpower to unfreeze himself to protect Aqua from Vanitas. Then Vanitas unmasks himself and merges with Ventus, leading to a Battle in the Center of the Mind on a corrupted Dive to the Heart showing Vanitas opposite Ventus. Ventus fights knowing full well that if he destroys the χ-Blade, both of them will lose their hearts. But as he says, "anything to save Terra and Aqua". And the fight ends with the two using their Keyblades to fire a giant Beam-O-War, Ventus overpowering Vanitas to charge up his Finishing Move and deal the final blow.
Something particular to note about the whole Ven-Van clash: only when fighting Vanitas is Ventus well and truly pissed off. Recall Aqua's line: "What else is darkness but hate and rage?" Ven has a heart of pure light. Being in such close proximity with his former darkness, Ven manages to become empathically enraged.
As Aqua and Mickey encounter a spacey Ven, the former approaches him to ask what is going on. Mickey instantly realizes there's something fishy about the situation, so when "Ven" reveals himself to be Vanitas possessing the poor kid and immediately tries to stab Aqua with the χ-Blade, the King jumps in, blocking in with his Star Seeker when the χ-Blade was millimeters from Aqua's heart! It's right there you know King Mickey took one of his many levels in badass.
Also a crowning moment for Vanitas, in which he shows that he is not a stupid villain. This is the second time that he's attacked Aqua without warning, because he's faced her twice in the past and she won. He's the only villain smart enough to use sneak attacks.
And of course, there's the True Final Boss. You get to fight Xehanort again as Aqua, and he's just as powerful as when you fought him with Terra. Then he busts out the Guardian. And Aqua still kicks his ass.
Scrooge McDuck talked back to an Unversed. Sure, he's badass in his canon, but that still takes balls to do that while unarmed. All because he thought the Unversed may have been after his munny, of course. That's right, when faced with possible death, Scrooge only cares for the safety of his well-earned munny. Now that is Badass.
AnotherCool Old Guy from Disney has a great moment in Ven's story...
Ven: Mister, get inside! It's not safe out here! Merlin: Oh, ordering me about—now that's unsafe!.
Aqua creates Castle Oblivion. This place was the whole mysterious setting of Chain of Memories and it's almost a mindfuck when you learn that it was Aqua who created it. She modified a whole world!
Aqua, gameplay-wise, she canonically beats both of the other main character's final bosses. Hell, she took on Ventus's twice! And she beat the Memetic Badass, Vanitas, three times, with the last one being when he is in possession of the ultimate weapon!
Final Mix adds a substantial amount of Pureblood Heartless, including a Heartless apex predator, all of which are fought on their own turf in the Realm of Darkness to Aqua's kill count.
If you count the secret bosses, Aqua also fights: the Lingering Spirits of Vanitas, Eraqus, and Xehanort; a younger version of Master Xehanort possessing Time Master powers; an Unversed monster that loses more of its restraints after every fight, becoming stronger; and a whale. All of these are non-canon, of course, but it's an impressive feat for someone who just obtained her Mark of Mastery.
Vanitas has some impressive Crowning Dialogue of Awesome, though some of it may also count as Nightmare Fuel. Among the greatest examples for him is this: "In that case, I'll choke the life out of your friends. Then let's see how long you play the pacifist."
The scene where Terra is watching his home world being destroyed was pretty freakin' sweet. Not because the world itself was being destroyed (that's sad), but with the way the camera shook while he was just watching made the scene look intense.
After Ventus is frozen solid by Xehanort, he is dropped down to where Aqua is, who then has to fight Braig, and all he can do is watch. Then Aqua gets hit by Vanitas' sneak attack, and Ventus DEFROSTS HIMSELF WITH HIS ANGER to fight him.
"You may be my master but, I will not let you hurt my friend!" was an epic one for Terra, even if it turned out prettybadlyforhim.
A villainous example has to be when Braig, pissed that he got scarred from a fight that Xehanort set up, starts shooting at him. How does Xehanort take this? He just sits back and lets the bullets hit his shield. And then, when Braig gets in close range, Xehanort just summons his Keyblade right to his throat, causing Braig to back down his aggressiveness.
Xehanort's face during all this. It looks like he's just laughing off all his pitiful attempts to do any damage.
Another villainous example: when Vanitas explains how he creates the Unversed, he casually creates a whole bunch of Unversed, including Floods, Tank Topplers, and a Trinity Armor, and then just as casually dismisses and reabsorbs all of them. The first time through, those guys could beat you right up, but he doesn't even acknowledge it as even a thing.
The entirety of Aqua's journey through the Enchanted Dominion. Captured and thrown in a dungeon with Prince Phillip, the two fight their way out of the castle room by room, taking down hordes of Maleficent's goons, to confront Maleficent herself on the bridge to Aurora's castle. Then she turns into a dragon and Aqua and Philip work together to bring her down.
Ventus gets a minor one for the Enchanted Dominion: Ventus pretty much fights Maleficent - not the dragon, but pure Mistress-of-all-Evil Maleficent - in a one-on-one battle and wins. In the following cutscene, Maleficent, while not down, is actually worn out.
Aqua has to fight Hades during her visit to Olympus Coliseum and he uses the rules to his advantage to summon a fake Ice Titan as backup (Rule #2: Combatants may call for backup) and Zack tries to be Aqua's backup and she declines and takes them both on alone and wins. She took on a fake Ice Titan and a god and won.
Like two of the final bosses in KHII, the No Heart boss starts out sitting on his throne. Unlike II, he gets his ass out of that chair midway through! If you're not prepared for this he will wreck your shit.
One for Aqua that's not even in the story. When the Bonus Boss Mysterious Figure appears before Ventus and Terra and summons his laser blades, the two of them summon their Keyblades with a surprised look on their faces. When the same thing happens to Aqua, she just calmly summons her Keyblade like it's no big thing with a "let's do this!" look on her face.
Blank Points implies that Aqua has been aimlessly wandering the realm of Darkness of years pretty much nonstop. Made even more awesome in the Final Mix version which shows she's been constantly attacked by heartless that can very easily kill the player constantly this ENTIRE TIME and has somehow survived this. Aqua pretty much defines the word Determinator.
Blank Points also gives Eraqus a posthumous one: Before dying, he preemptively gave his Light 'em Up Heart to Terra to later counter Master Xehanort's Casting a Shadow one — proving why MX can't touch Terra's Heart despite having his Body. Even MX's impressed:
MX: "Oh? So that's how it is, is it? Someone else has set foot in your Heart. Eraqus, you sly fox..."
The Fairy Godmother stopping Aqua from presumably murdering Lady Tremaine and her daughters by the young Keyblade Master that light and darkness go hand & hand, and cannot exist without each other. (Even though it turns out to be a Broken Aesop, but still...)