Reply 1988
Coffee Prince
Master's Sun
Kill Me, Heal Me
it has to be Mary Stayed Out All Night closely followed by You're All Surrounded.

I am really loving the way they're using music in Moon Lovers.

and even though Missing You is close to my least favourite drama ever, I did like their use of music too.
also + 1 for Iljimae, Reply 1988 and You're Beautiful
Bad Guy (Thorn Flower♥)
Dear My Friends (both songs and BGM)
Chuno, absolutely everything ♥
Iljimae
Time Between Dog & Wolf
The Princess' Man
Bad Guys (mostly BGM)
My Girl

I love these OST's:
M.C The Max - Because It's Love from Sensory Couple a.k.a. The Girl Who Sees Smells
Lee Seung Chul - I'm In Love from You're All Surrounded
Kim Yoon Ah ft. Olltii - Lucid Dream from Missing Noir M
ALi - I Love You, I'm Sorry from Angry Mom
Winter Sonata      Ryu (all the songs he sang for WS ost album) 
My Sassy Girl      Shin Seung Hun- i believe (um duh this song is a must in this list)
Bad Guy       Kim Yeon Woo-sometimes i cry alone
A Gentlemen's Dignity     Lee Hyun-My Heartache      Kim Min Jong- beautiful pain
My Lovely Girl    Kim Tae Woo-i only want you
Doctors (2016)   Jung Yup- its love
Secret (2013)      Eru- secret
Second To Last Love   Ben- my all
Shine Or Go Crazy    Ailee-are you the same
Lovers (2006)    Jo Jang Hyuk-lovers
Kill me heal me    Jang Jae In- auditory hallucination      Ji Sung-violet

etc... 

School 2015
DOTS
Dream High
My Girlfriend is a Gumiho (Love Fox Rain)
Secret Garden
Marriage Not Dating
I Can Your Voice
Boys Over Flowers
The ones I remember atm
heartstring
you're beatiful
DOTS
You're Beautiful
Secret Garden
Marriage not Dating
It's Okay That's Love
Pinocchio
Right Now I love these songs-
Stay With Me, Who Are You(Goblin)
This Love, Everytime (DoTS)
Wind Flower (LOTBS)
Love's Forever (Marry Me Or Not)
Best 2016 Kdrama osts are from descendant of the sun, goblin and moon lovers
My 2 top favorite
1: Legend of the blue sea.
Every time I watch it I feel overjoyed.
2: You who came from the star.
I love it, but it saddened my heart the way it ended. I always wish there was more episode.