When Honey-sempai swung in from a vine and kicked a fully armed, armored, and dojo-trained private soldier in the face. Complete with Tarzan yell.
Kyouya has a couple of such moments, but the greatest one is in the final episode when it's revealed that he not only bought out his father's company but after doing so, he gave it back to him because he'd found something more important in life. Simply glorious.
In the manga, while the second-year students (minus Tamaki) take a class field trip to France, Kyouya spends the entire time searching for Tamaki's mother so that her son would know how she was doing. Kudos to you, Shadow King.
Haruhi's final moment of badassery. Here's the lowdown: Tamaki's spiteful grandmother has effectively blackmailed him into marrying a French girl named Eclair Tonerre. He is in the backseat of a convertible, speeding along a bridge, missing the final events of the school's festival and torturing himself with the idea that the Host Club members all had to make tremendous sacrifices just to satisfy his whims. Out of nowhere, a horse-drawn carriage leaps onto the road with a mighty crash, and proceeds to keep up with the convertible—it's Haruhi, in a billowing white dress, there to bring her senpai back. She shouts that Tamaki has been an invaluable friend and that the Hosts love being a part of the club, and indeed that she loves it as well. But that's not all! Tamaki then proceeds to have his moment, when the horses suddenly run wild and Haruhi is thrown off the bridge. Seconds later, Tamaki is plummeting above her, having hurled himself after her. He grabs her in midair (in romantic slow-motion, no less), cradles her to him, and executes the loveliest swan dive into the water below.
And the moment Haruhi takes off in the chariot without the twins, the ending song, "Shissou", starts and doesn't end until both moments have been completed.
The whole of the last episode seems to be one giant Crowning Moment of Awesome.
Indeed it was. For an anime that seems to be mainly comedy, the last episode was incredibly awesome. Another notable moment was Honey and Mori leaping into battle with a large squad of the private police, with Honey casually telling Mori to not go easy on them.
Kyoya gets his own in that episode when he slams his fist down on to his own limo and leaves a huge dent. Then, when Haruhi is unsure of whether or not to get on the carriage with the twins, he takes her shoulders, tells her to "bring that idiot back," and then proceeds to tell Kaoru the fastest way to catch up to Tamaki. All this, in direct contradiction to his father's orders, when Kyoya was previously shown to always do what his father wanted.
I think it starts with Haruhi calling out Kyoya's father and telling him in no uncertain terms that he's wrong about Kyoya (and apparently earning his respect in the the process.)
Honey's battle with his father proves him to be so deadly that he is forbidden from ever fighting at full capacity ever again. Why, you ask? Because, as Tamaki explains, other countries would begin to think that Japan was harboring weapons of mass destruction if Honey were turned loose. That's right. Honey is a huggable, bunny-toting, shota-riffic human nuke.
Yes, but Tamaki may have been exaggerating a little. Just a little.
Manga only, defeating Honey in a duel in chapter 72. By grabbing him mid-flying kick and gently placing him out of bounds. And then demanding that he not bring his bunny to university.
You've got to admit, slashing through a potted plant in mid-air with his bare hand to save Kasanoda (episode 22) practically solidified him as the show's resident Badass.
Nekozawa spending a whole episode scaring everybody. May not seem like much, until you consider that Haruhi was previously shown to be scared of NOTHING outside of thunderstorms. Anyone who can make her scream in terror is pretty darned awesome.
Not just Haruhi, either. He sent the entirety of Class 1-A, including Haruhi, the Hitachiin twins, and Renge, running like frightened squirrels... with a hand puppet.
On a more serious note, there was also the fact that he ran out into direct sunlight to help his little sister, despite the fact that direct sunlight causes him to collapse. Aww.
In the manga: Kousaka, a lawyer working for Tamaki's grandmother and a former neighbor of Haruhi's father, visits the Fujiokas' home to present Ranka with an offer for Haruhi to study abroad - an arrangement set up by Grandmother Suou to keep Haruhi away from Tamaki. Ranka hears Kousaka out... and then throws a cup of tea in her face and tells her off for being a part of it, telling her not to take him for an idiot and stating that if Haruhi had any desire to study abroad, of course he would have prepared for it somehow.
Chapter 76 provides one for Tamaki's dad. Screwing over his own over-domineering mother, who's been making life hell for him, his son, and the woman he loves who is also the mother of said son. A plan years in the execution performed flawlessly to render her ability to have any say in their lives powerless. Now if only Tamaki would stop being so emo about missing his chance to reconcile with his grandmother.
Kyoya's utterly EPIC verbal beatdown of Tamaki's father after the man doesn't understand Tamaki's sorrow at what he's done, is pretty awesome too.
The twins, episode five, revealing that they're Magnificent Bastards on Kyouya's level. Let's see, they overreacted to a small comment and got into a huge fight that Haruhi felt so bad for causing she promised to let them into her house if they'd agree to apologize. That or they're the fastest thinking mofos on the planet.
Chapter 80 is, like the finale of the anime, one huge Moment of Awesome. It's topped off with a Crowning Moment of Heartwarming as Haruhi finally tells Tamaki she loves him.
In chapter 80, so it might not need to be here, but the host club fangirls helping Tamaki get to the airport. For teenage girls, that's insanely badass.
The episode showing how Kyouya and Tamaki meet had a moment where we see the symbolic picture Kyouya has been painting. Up to now, Kyouya has stayed within the thick frame, but the final view shows an odd smear leading out. Pan back and a huge rose is revealed above the painting.
In episode 26, with Honey and Mori performing a You Shall Not Pass moment on the Otori Group's police force.
Honey: Takeshi...DON'T GO EASY ON 'EM!"
Yeah, it turned out Haruhi wasn't in any danger after all, but you gotta admit the host club boys "saving" her from the doctor early on in the series is AWESOME. It's not just Tamaki or Honey, it's ALLSIXOFTHEM, dropping everything and running into the clinic to beat the ever-living heck out of the guy they think is about to hurt their princess. And this is only the third episode.
Hikaru/Kaoru: One: Good looks that attract the public eye.
Kyouya: Two: More wealth then you can imagine.
Mori: Three: Chivalry that will never be able to overlook-
Honey: -the hideous wickedness of this world.
Tamaki: *Quickly drapes his shirt over the undergarments-only Haruhi* That's what makes up the Ouran Host Club!
All: We're here! WATCH OUT.
During the Sports Festival chapters, Tamaki's team is being crushed by Kyoya's and want to just quit. Tamaki revitalizes their morale with a Rousing Speech in the form of a monologue from Henry V. In full costume.