That's on my "to watch" list; my friend's been watching it and telling me how good it is.
My favourite so far is probably the first K-drama I watched, which was Are You Human Too?; I've actually already watched it a couple of times.
I've also watched Circle: Two Worlds Connected (which I'd love to see return for another series), Sky Castle, He Is Psychometric, and I'm currently working my way through Her Private Life. I haven't been disappointed yet...
I've seen mostly mainstream k-dramas, so out of those I really like Strong Woman Do Bongsoon and Goblin. My mom (huge fan of k-dramas) also recommends You Were Beautiful and Bride of the Century (she loves Hongki from FtIsland).