The best OST I've heard is Resurrection's. I've seen quite a lot of dramas and none of them had this good OST songs.
My favorites from Resurrection :
(there was a very, very well made fan video for this song, but unfortunately it has been deleted. It even had lyrics.
And, of course, the main theme song. I'm not a big fan of classical music but I love the song. The original song(OST) isn't in Youtube but there is a orchestra cover, and still, it's just superb.
Few other Kdrama OST songs that are good(I'm a fan of sad OST songs, because they carry a meaning and feeling)
A Love To Kill OST. The song is called Dream and sang by K.Will. This has eng subs(for better understanding) but this video doesn't have last 30 seconds of the song. :c
What Happened in Bali OST song. Very good song.
This is pretty much the only OST song that I like but is on a positive note. Suzy's tender voice is lovely and I liked the lyrics, too. They sounded somehow familiar, not sure why. I may be wrong but there may actually be a similar song in Latvian.
These aren't the only ones I liked and are good, there are few from The Devil, My Girl and so on.
And while searching for these songs, I found a very, very good OST song from Brilliant Legacy. Just found it and I love it already. I feel like watching Brilliant Legacy again. ;D