There are so many good ones but the most recent is Jisu - Hold On from Marriage Contract
-Dream High- Love high
-The Heirs- I'm Saying
-You're beautiful- Still (As ever)
-Moorim School- Alive
Kill Me, Heal Me (Jang Jae In - Auditory Hallucination )
Who Are You: School 2015 (Tiger JK - Reset)
Empress Ki (Xiah Junsu - I love you)
Descendants of the Sun (Mad Clown - Once Again) 
Secret Garden soundtrack. literally all,
Everything from Age of Youth are amazing
Most memorable:
1. East of Eden  --- Every Single Song ... lots of ballads, Lee Seung Chul, SG wannabe, n more .... https://www.youtube.com/watch?v=ASrIn8OUtec

2. Chuno  ---  Especially:
 "Stigma" by Yim Jae Beom:  https://www.youtube.com/watch?v=c5N2ruT70Ww&list=R  ... ...  
"Bakkwo (Change)" by Gloomy 30's:  https://www.youtube.com/watch?v=mTx4I-DGY0Q  ....
EopBok The Sniper's song 민초의 난 by MC Sniper:  https://www.youtube.com/watch?v=KujRinYsxzk  
"Lost Child" by JeA ... "Dale Jadi (Moon Has Passed)" by Beige  .....  "Biingnyeolii" instrumental  ...etc...

3. The Princess' Man .... Especially: 
"Vola" : https://www.youtube.com/watch?v=LvMsVnHhNJY
"Destino": https://www.youtube.com/watch?v=pPxcDTT3lKs  
"Again, Today I Love You"  by Baek Ji Young: https://www.youtube.com/watch?v=5uI8i55kqIY
..... "Goodbye My Love" ... Longing(회모) ... A Day ... etc...
Eternal Love - Michael Learns To Rock (OST Healer )
There are many beautiful songs in each drama but for me OST means all the songs in it generally . My favorite drama OSTsit's ok, that's love , descendants of the sun ,six flying dragons and secret garden.
Descendants of the Sun has one of my favourite soundtracks of all time
You're Beautiful
Two Weeks
Heartstrings
Kill Me, Heal Me
My Love From The Star
Pinocchio
School 2015
DOTS
Chuno
Pasta
Coffee Prince
Iljimae
Kill me, heal me
Time between dog and wolf
Reply 1988
Secret Garden
Bad Guy
Who are you: School 2015
Secret Garden
Cruel City