Mizuki's various skills such as becoming animals, shape-shifting into other people, and becoming a Dragon in the finale.
Kai and Oishi's escape from the Dutch
Kai versus The Tengu Master in their own turf. He managed to grab the sword faster than the Tengu Master and damn near decapitated him.
Mizuki, Lord Kira's second in command, disguising herself as a prostitute and seducing one of the ronin working for the heroes, leading them right into a trap that nearly killed them all in one fell swoop is one scene of villainous awesome.
The 47 Ronin's attack on Lord Kira's heavily guarded castle is one Crowning Moment of Awesome, especially during the battle scenes.
Oishi's killing of Lord Kira in the finale of the film, finally avenging his lord.
Kai versus Mizuki (who is a witch) in the final battle scene, which the latter becomes a Dragon and put up a heck of a brutal fight against Kai, nearly killing him quite a number of times. Kai wins in the end though.
Heck, even the heated exchange before the fight scene was awesome: