I like alot of songs from the Scarlet Heart: Ryeo and Descendants of the Sun OST's, so probably those 2 albums in general.
Heartstrings
Moorim School
Jelousy Incarnate
Healer
Who Are You: School 2015
Bad Guys
Beautiful Mind
korean drama ost's often have such a nice sound to them and sound so emotional so it's really hard to pick but my favorites have to be:

roy kim - pinocchio (pinocchio)
yoon mi rae - i'll listen to what you have to say (school 2015)
and of course jin & v's - even if i die (hwarang)
Moon Lovers: Scarlet Heart Ryeo
The Princess' Man
Kill Me, Heal Me
Empress Ki
Descendants of The Sun
Doctor Stranger
Secret
Boys Over Flowers
Queen of Ambition
Innocent Man
Descendants of the sun 
Goong
Rooftop prince
You who came from the star
Strong Woman Do Bong Soon OST:
Every Single Day - Super Power Girl
Standing Egg - How Would It Be

Hwarang OST:

V & Jin of BTS - Even If I Die, It's You
Wendy & Seulgi - I Can Only See You

Introverted Boss OST:

Ben - Memory
Hong Dae Kwang - Is it Love?
Sandeul - One More Step
Park Bo Ram - Isn't She Lovely

Cinderella & Four Knights OST

Green Cacao - If I Meet You Again Someday
Dick Punks - You Pour A Star
BtoB - For You
SinB - Confession

She Was Pretty OST:

Kim Min Seung - Thumping
Ki Hyun - One More Step


Others:
Acourve - Honey (Sensory Couple OST)
Crush - Beautiful (Goblin OST)
Jang Geun Seuk - Without Words (You're Beautiful OST)
Jung Joon Young - Where Are U (W - Two Worlds OST)
Kim Feel - Tomorrow With You (Tomorrow With You OST)
Oh Yoo Joon - Love Has Started (Glorious Day OST)
Kim Min Seung - From Now On (Weightlifting Fairy OST)

You Are Beautiful sure has some nostalgic tunes. Also Goblin!
currently I'm enjoying the OST of My Secret Romance by Sunghoon and Song Ji Eun
My secret romance - same
Goblin - stay with me
full hous (thai) - oh baby i
kiss me (thai) - kiss me
hwarang - a move of god
those are just a few kkkk
My favorite OSTs are
It's Okay, That's Love
Kill Me, Heal Me
Age of Youth 
Descendants of the Sun
Another Miss Oh
Sprout( the only good thing about this drama was the music xD)
Introverted Boss

Coffee Prince - really eclectic, introduced me to some Korean indie bands which I love
Pinocchio - a good mix of rock and ballads

... and even though it's not complete yet, I'm loving the Chicago Typewriter OST sooo much  
Dream High
Shut Up! Flower Boy Band
The Liar and His Lover
Ooh Liar and His Lover was pretty good.

Dream High 
Dream High 2
You're Beautiful was amazing!!
DOTS was priceless
I like some of Goblin's OST songs
Currently, I am in love with Kim Min Jae's song in The Best Hit. Idk it's just stuck it my head and I love the message 
Healer's instrumental OST song

The Best Hit - Dream by Kim Min Jae ft Younha.. the message of the song and rap.was so good
Goblin - all OSTs... I still have the full playlist of it
Moon lovers scarlet heart ryeo - All OSTs... I also have the full playlist