My favorite K-drama of all time is "Bridal Mask"
It came out in 2012 and here is a little more information:
Synopsis
In the 1930s, Lee Kang-to lives in Korea, which is oppressed under Imperial Japanese rule. Despite being Korean, he works with the Japanese police in order to capture "Bridal Mask," a mysterious rebel who fights for Korean independence. Later, he secretly dons the Bridal Mask and fights the unjust Japanese regime. He struggles to balance his personal facade as a Japanese police officer and his private life as Bridal Mask.
Kimura Shunji is a gentle Japanese schoolteacher. He is best friends with Kang-to and falls in love with the fiercely patriotic Mok-dan. His hatred for Bridal Mask and unrequited love for Mok-dan twists him into a darker person. -Wikipedia
I would highly recommend K-drama fans to check it out.
What is your favorite K-drama of all time?