I've watched Mask a long time ago, but this song is very unforgettable.
Also, it's nice that some of my faves are also getting the attention they deserve (especially Auditory Hallucinations - Jang Jae In).
There are undeniably a lot of great osts out there, some of which I may have never heard yet but, in my opinion, Moonlovers: Scarlet Heart Ryeo's OST got to be one of the best! I'm emotionally attached to every song (most probably because of how much I cried while listening to them) but my top three faves would be: [not in order] Forgetting You - Davichi, My Love - LEE HI, and Can You Hear My Heart - Epik High, LEE HI
Other Songs I'm currently loving: I Open My Eyes (Just Between Lovers), Satellite (Chicago Typewriter), and Can't Go (Because This Is My First Life).