Saya's first fight in Episode 2. Seeing her easily take down that Chriopteran is but a taste for how much more awesome action is yet to come.
The end of Episode 33. After the Time Skip, Lewis and Kai are trapped by not one, but four Chiropterans. Given that their guns are not effective against these monsters, one would think they are finished. Low and behold, Haji's case comes smashing in the street and the next thing we see is Saya, returning after a full year and prepping her sword for battle. In short, after a year of absence, She's Back!
which leads to the beginning of Episode 34, where she easily makes short work of the Chiropterans. Given her much improved fighting skills, it's hardly a surprise why so many thought the series stepped up from that point onward.
Saya finally giving James exactly what he deserves in Episode 47.
The Final Battle (two to be specific) between Amshel vs. Hagi and Saya vs. Diva. In all sincerity, amazingly animated and choreographed fights by any means, not to mention cathartic since Amshel gets deep-fried on the tip of a skyscraper. Diva's death, on the other hand...
Hagi taking Saya's sword to deliver the finishing blow to Amshel. After 50 Episodes, Amshel at long last pays the price for the shit he has done throughout the series.