Vita's first appearance. All the giant hammers that followed can't really compare to the sheer badassery of busting through Nanoha's shields, staff, AND armour, and then knocking her through a wall. Not to mention this is the first time we get to witness the Cartridge System at work.
The Big Damn Heroes in the first episode done by Fate and Yuuno. Nanoha about to face death by hammer when suddenly a green magic circle appears, Fate's cape fluttering as she blocks Vita's attack all with her theme playing.
So Fate and Arf had immobilized Vita and is about to interrogate her when suddenly, a pink haired woman shows up, knocks Fate back, sets her sword on fire, slices Bardiche like it was made of glass and sends Fate several floors through a building. Ladies and gentlemen, I give you Signum.
Nanoha taking an attack from Vita while in civilian form... then calmly walking Out of the Inferno that it created, before uttering the line that gave birth to her Fan Nickname.
Vita: "You devil..." Nanoha: "It's okay if I'm a devil... it just means I'll have to use my hellish tools to get you to listen!"
Chrono finding out the The Plan perpetrated by Admiral Graham, then curbstomping his two teachers of close combat and magical combat, Aria and Lotte. He does it with Struggle Bind, a less-used spell that not only binds the target, but also dispels enchantments like transformation spells. "You're the one that told me to keep practicing", indeed.
There's also the Kirk Summation he gives to Admiral Graham and the Lieze Twins, saying that their plan to sacrifice the Book's Master to freeze it away forever is wrong because 1.) It involves an innocent girl rather than a criminal and 2.) It's possible for people to retrieve the book from where they're planning on hiding it. Lotte seems to be the only one willing to try to argue with his points, and Graham quickly shuts her down, most likely realizing that they can't convince him to see things their way any longer.
The entire battle with the Book of Darkness, in particular:
Reinforce taking on all four main characters at once, with them on the defensive? Badass. The attitude she pulled it with? Awesome. Nanoha pierces her barrier to deliver a point-blank Exelion Buster. Doesn't. Do. Shit. Captain.
Nanoha's response? "I'll just have to try even harder". A.C.S. Standby. The entire scene was so full of Nanoha-style GAR.
Snow Rain playing in the background as Hayate takes control of Reinforce and lowers the defense just enough so Nanoha can blast her with full power while Fate busts her way out of the Dream World at the same time. Everything about this scene is just perfect.
Hayate and the Wolkenritter together after Hayate is recognized as Mistress of the Night Sky and she resummons the 4 knights. They then all say awesomely epic lines and Hayate transforms.
Signum: "We are the the guardian knights who gather before our mistress, she of the night sky."
Shamal: "As long as our mistress exists, our souls will never extinguish."
Zafira: "As long as we have life in our bodies, we will stand by each other."
Vita: "We will always exist before our mistress, Queen of the Night Sky, Yagami Hayate."
"Reinforce, bestow upon me my staff and armor." The realization that after having resurrected four characters, Hayate the ill girl will fight alongside the heroes was awesome. Followed by the just as cool line, "Light of the Night Sky, come to my hand! Blessed Wind, Reinforce, SET UP!" Really all of Hayate's overly-long incantations sound cool.
The final battle against the corrupted defense program, in which each one of the heroes fires their most powerful attack with the monstrosity barely having time to regenerate itself, let alone fight back, before being hit again. Culminating with:
Nanoha: "Full power! Starlight..."
Fate: "Lightning flash! Plasma Zanber..."
Hayate: "Resound, horn of judgement! Ragnarök..."
Shamal locates the core and she, Yuuno and Arf transport it into outer space so the Arc-en-Ciel can be fired at it. Cue massive fireworks.
The A's manga has a game of dodgeball in one chapter with Nanoha and Fate playing against Alisa and Suzuka. Nanoha sucks at sports, but with Fate around there should be no contest, right? When Nanoha gets hit she leaps about fifty feet into the air to catch the airborne dodgeball, stunning every muggle in the immediate vicinity. While still in the air, she chucks the ball down with blinding speed at Suzuka... who catches it with one hand, chucks it back at Fate and hits her hard enough to knock her out of the sky and unconscious. With a dodgeball.
The MOVIE 2nd A's
After Nanoha and Fate get their Linker Cores drained, they still want to train, but can't use any magic for a while and don't have their Devices, so they have a practice fight with staffs. What makes this better is how completely out of nowhere it comes, since Nanoha has never cared much for Full-Contact Magic. And yet she is somehow able to match Fate, which suggests she may not be entirely dismissive of her ninja princess status in this continuity.
Nanoha vs. the Book of Darkness revamped:
Activating Exelion Mode is so awesome, that not only does it transforms Raising Heart, but also restores Nanoha's Barrier Jacket, turns the vest part black, and makes her hair ribbons bigger. And A.C.S. now has eight wings and the Strike Flame is as long as Nanoha is tall, making it one mean looking lance.
Remember that scene where Nanoha uses Raising Heart's Exelion Mode to get through the Book of Darkness' defense for a point-blank Exelion Buster? Here she uses the same trick, but pushes the Book of Darkness, who was showing a bit of effort trying to stop the attack, THROUGH SEVERAL ROCK PILLARS. Nanoha finally pins her on a large rock pillar, then manages to get through and shoot point blank, DESTROYING THE ROCK PILLAR SHE PINNED THE BOOK OF DARKNESS ON. What happens next? The Book of Darkness rises from the ruins without so much as a scratch. GOOD LORD!!!
And then Reinforce proceeds to wipe the floor with Nanoha. Not with magic or anything, she just smacks her around with her bare hands. At one point she grabs her by the foot as she's trying to fly away and pounds her into the rock pillar behind her. You don't often see that in aerial combat. And when Nanoha still refuses to stay down, she binds her and summons a giant friggin stone drill above her, just to show how pissed off she is.
The final battle of course. With much better animation.
Sturmfalken looks much more epic. Well, all attacks look more epic.