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It's Okay that's Love has a great OST
Fated to Love You (Korea) and Another Miss Oh also have beautiful OSTs. Secret Garden's OST is pretty famous ofc.
My Love From Another Star and Good Doctor actually have really beautiful scores as well.
It's Okay That's Love - you can't miss this.
Reply 1988 - a lot of old songs but they're the best.
I love the background music in Reunited Worlds, it kind of sounds fairy-tale-ish
Also a a drama with beautyful OST was Masters Sun

other genre: Faith and Gu Family Book
City Hunter has amazing OSTs AND soundtracks!

then we have
Heartstrings
To the beautiful you
Kill me heal me
Dating Agency Cyrano has a ballad kindof ost that I love
Strong Woman Do Bong Soon
Monstar
Shopping King Louis
Moorim School has an OST by Vixx which is awesome
Fated To Love You has an Ailee OST which is great too
Please Come Back Mister only has good OSTS. You should check them out- http://mydramalist.com/15958-please-come-back-mist...
I personally like Again by Noel- https://youtu.be/ymLHpHqGU6k
Legend of the blue sea    
-somewhere someday     by Sung Si Kyung
-you are my world      by Yoon Mi Rae
-wind flower      by Lee Sun Hee


My love who came from the stars   
-every moment of you    by Sung Si Kyung 
-my destiny    by Lyn

Secret garden    (lol ofc along with the other 2 drama ost's above XD) 
-you're my spring   by Sung Si Kyung
-that women/that man   by Baek Ji Young
-here i am           by 4men
-watching     by Yoon Sang Hyun
-appear        by Kim Bum Soo
-I cant          by MIII
-here i am    by Yoon Sang Hyun
-reason        by Shin Yong Jae of 4men

Winter Sonata
Please come back mister
Shine or go crazy
Kill me heal me
Fated to love you
Another ms oh


Sung Si Kyung, ost king XD

(only linked the ones not as well known or the ones by si kyung oppa)
Heartstrings, Cheese in The Trap & Shut Up Flower Boy Band.