Shifting Worlds/Awesome

After Aurion finally breaks free of Cichol's hold on him and completes his Hell Face Turn, both Aurion and Kanako Imrah (who overrode Jonathan's mind just for this very moment) unleash a double Indignation onto Cichol that actually makes Cichol bleed. That's right: A Milletian and human (technically a human using the body of a half-vampire) made the God of the Fomors bleed. Even Morrighan was impressed.
When the Black Spawn enters Gensokyo, it comes with some sort of barrier that renders any damage dealt to it null and void. No one's attacks did anything to it. No one's, that is, except for SDM's god-aura powered Megaton Punch. Yes. A punch completely broke the Black Spawn's invincible barrier and rendered its form unstable. Commence wailing.
In the Book arc, Cirno first using her One-Winged Angel form to defeat the dark youkai Rumia, Reli and their fusion form Relia. Quite an impressive feat, considering that just before the two were completely unable to even cast any spells from the corruption of Rumia and Reli's Dark Embrace, and that Rumia has effortlessly mauled Kanako (not the goddess) to pieces.
- Speaking of the book arc, the entire battle against Numb-whisper's final form, especially the Windseeker showing up to help.
Say what you will of Desmond, but his treatment of the Queen of Hearts after she mutilates Animus is worth a mention. Animus' own payback is not bad either.
Arachnos' reaction upon figuring out how Future Flandre has been manipulated by Hell gate, full of cold, contained, yet unstoppable fury: "Demonic. Twisted. Evil. Sick. Perverted. House." has become legendary.
The verbal clash, in Crossroads, of EX-Rumia and Remilia, the worldviews of a feral cannibalistic youkai and a refined, elegant vampire colliding. Specifically, mostly just about anything Remilia says. Rumia does regain her composure a bit later into the "duel", but Remilia mostly retains the advantage throughout. Hey, the very fact that she dares speaking up against EX-Rumia is Badass on its own, seeing as she's essentally insulting the embodiment of all darkness.
Meta example: the arrival of Hellgate 2 Part 3 practically singlehandedly rejuvenated the club, turning completely inactive people active. The roleplay hit about five-six pages in barely a couple of hours.