One word - Masterpiece.
I have seen so many historical dramas, but The Rebel Princess has set the bar for cinematography and screenplay. It's literally like watching a mini movie per episode.
Spectacular acting from the cast - nevermind that they look older than the role they're portraying. They pulled off the characters and the depth of emotions shown could not be easily replicated.
The development of each and every character was also applaudable. The way their relationships (family or kinship) complicated and affected their decisions is very realistic.
The chemistry of the the main leads is also something unexpected but has ultimately made this drama even better.
Oh and if you're looking for the typical whimsical, slightly unrealistic and cringe-worthy, light Wuxia Drama, this may not be it for you.