The "White Devil" training incident, where Nanoha gets disappointed and catches a magical blade with one bare hand, stopping the wielder in mid-air. And with the other, she catches the fist of the team's Hollywood Cyborg without so much as flinching. And then proceeds to produce Beam Spam from her fingertips without any assistance from her staff.
Nanoha's dialogue when Subaru mentions being worried about Nanoha's state of mind after Vivio has been kidnapped.
Nanoha: "Don't worry. The Nanoha you've always admired is an Invincible Ace who won't lose to anybody."
The first time we ever see Hayate in combat in StrikerS. Right after Fate offers to release her limiter to take on all the flyers, Hayate comes in and tells Fate that SHE will release her limiter taking her from an A class mage to an S class. She does this, then unleashes the mother of all laser attacks in Hręsvelgr, which is essentially a huge 5-barrel cannon that fires exploding shots. It doesn't get much more badass than that. And she's still not at full SS class power.
Nanoha and Fate have the Numbers essentially stuck between a rock and a hard place as their two giant lasers are coming at them from both sides. Then they notice Fate ceasing her pursuit and wonder what's going on. And then it cuts to Hayate in the sky, casting a very ugly spell that nearly everyone forgot that she still had: Diabolic Emission. Even though the Numbers escape, their reaction makes it priceless.
During the final battles, Shamal and Zafira effortlessly locate the strategist Otto, annihilate her entire security detail in seconds, and trap her. Especially satisfying considering that Otto (with help from Deed and a small army of Gadget Drones) had blasted them into oblivion the last time they fought. Revenge is ever so sweet.
Subaru's fight with her brainwashed sister Ginga at the end of StrikerS was extremely badass, if for no other reason than having two Wing Road spells criss-crossing while the riders took turns punching at each other. Even Subaru's Device Mach Caliber has a Crowning Moment of Awesome when Subaru was knocked unconscious, and Mach Caliber yanked her body around so it could keep fighting until she came around. Complete with nonchalant: "Just as rehearsed" when Subaru wakes up. The coup de grāce that seals the awesomeness is how Subaru finishes it, by going head to head with the person she's lost to every time so far.
In the final exchange, Subaru's defenses give out, and Ginga's strike destroys her headband and leaves a nasty gash on her forehead... just before her defenses give out as well, and Subaru's Divine Buster proves the truth of her words:
Teana gets isolated by three members of the Quirky Miniboss Squad. After getting crippled and using shoot and run tactics against them for most of the fight, she baits all three of them to her last stand and proceeds to counter all of their attacks, smoothly taking them out one by one. When after dropping two of the Combat Cyborgs with ease she raises one of her guns and points it at the only remaining Combat Cyborg to quite calmly state "You're under arrest."
Sister Schach's victory over Sein. Sein phases through the ground under Schach and grabs her ankle in an attempt to immobile her so that she'd get crushed by a falling Type III Gadget Drone. Schach, without skipping a beat, immediately uses her Device to blow up the very ground she's standing on to escape, sending them both crashing several floors down. The best part is Sein's expression of pure shock and fear afterwards, as she realizes that she's now alone with a Belkan Knight who's battle-crazy to do that just to get at her opponent.
Despite being a Mad Scientist and Big Bad, Jail Scaglietti still deserves credit for displaying his brilliant Slasher Smile even after Fate pulled herself together, beat his bodyguards down and attack him with her BFS. You can say what you want about the man, but he kept his style until the end!
Quattro's face was the page image for Oh Crap for years (and has been reinstated as the Anime and Manga page image) for a damned good reason.
And the way that she ended up finding Quattro? A little spell called Wide Area Search, which she used in the very first season.
Signum and Agito in the final episode. Two words: "Karyuu Issen". It turns her sword into a very long ass Flaming Sword and in one swing, she destroys tons and tons of drones.
The "After Days" special manga chapter. Signum vs. Nanoha, no holds barred. In its final moments, Signum takes the full force from a new attack from Nanoha in Blaster Mode 1 then, one-upping the famous Out of the Inferno moment of Nanoha, remains in the ensuing flames like the total Bad Ass that she is and uses it as a cover so she can set up Sturm Falken and shoot an unprepared Nanoha out of Starlight Breaker. Good God, Signum! Nanoha still managed to get a draw somehow although she was dizzy and on the ground after the last exchange of blows, while Signum was not.
Vivio is currently the holder of the title of "Most Befriended" by taking not one, not two... but FIVEStarlight Breakers and being able to stand up afterwards. Her awesomeness and Moe factor were so off the charts that Nanoha decides to adopt her afterwards.