Issun's Big Damn Heroes moment after Yami electrocutes Amaterasu to near death.
The final battle against Yami. You finally regain your former glory, becoming Shiranui, including the awesome wisp things and markings. Even your brush gets a makeover! It just fills you with pride and happiness that you can finally defeat Yami. It also couples with the crowing moment of heartwarming because it turns out Issun hasn't abandoned you. It makes how long the game is even more worthwhile.
Also, if you use specific brush techniques, Queen Himiko, Nagi, the real Rao, and Oki(although he isn't dead) all help you out. Even the dead help you out.
Think all the bosses (even the Spider Queen) are awesome? Ninetails countered your Celestial Brush, Lechku and Nechku required you to team up with your PAST self and OKI. Oki is a WOLF in this one. Daaang.
After the fight with Orochi, Susano finally does something useful! He deals the finishing blow, without any timely Power Slashes from Amaterasu, and rescues his girl. Good for him.
Even before that, Susano comes and confronts Orochi on his own, and rejects the monster's offer of power. Considering that he spent his time trying to distance himself from his legendary ancestor's reputation, it's a bold step for him.
You also get to see his ancestor Nagi in action. Sure, he was just as big of a Butt Monkey as "Pops", but the constant set-backs didn't deter him from making the same dramatic entry and killing blows on True Orochi.