You and I - Kim Jong Wan of NELL (Weightlifting Fairy)

Words My Eyes Say - Tei (Healer)

To You - Sung Si Kyung (Reply 1994)

Confession - DeliSpice (Reply 1997)

Where Are U - Jung Joon Young (W)

Star - Kang Min Hyuk (Heartstrings)

Even If I Die, It's You - BTS V and Jin (Hwarang) (I haven't watched Hwarang though. Hehe)

I have SO many but these are the ones I love; I can listen to them every day.

I absolutely love whole OST of Shut up Flower boy band. its been few years but i still have it in my playlist.
Recently i have also fell in love with Age of Youth OST, its seriously so good. As for a single song from the drama, I still from time to time like to listen It has to be you by Yesung from drama Cinderellas Sister :D
Jang Jae In - Auditory Hallucination (Kill Me, Heal Me)
The Once - You're My Best Friend (It's Okay That's Love)
BMK - Hey Buddy (Warrior Baek Dong Soo)
Lyn - Want To Be Free (Dear My Friends)
Ali - Hurt (Rooftop Prince)
Tiger JK - Reset (School 2015)
Soyu - Just Once (Empress Ki)
Epik High - Can You Hear My Heart (Moon Lovers)
kim minseung - from now on (weightlifting fairy)

kim jongwan - you & i (weightlifting fairy)

han heejung - dreaming (weightlifting fairy)

standing egg - i'll pick you up (weightlifting fairy)

lee jin ah - again again again (weightlifting fairy) 

davichi - forgetting you (moon lovers)

epik high ft. lee hi - can you hear my heart (moon lovers)

lee hi - my love (moon lovers)

tiger jk ft. jinsil - reset


i listen to them at least one day 

Way too many to name, some dramas I think I liked 99% of their OSTs. I’ll randomly name the top 10 that I can recall this moment:

1. Run – NELL (Two Weeks)
2. Cross My Mind – Twin Folks (It’s Okay, That’s Love)
3. Lovesome – Ra.D (Jealousy Incarnate)
4. Maybe I – Roy Kim (Another Miss Oh)
5. Who Are You – Sam Kim // Beautiful – Crush (Goblin)  I honestly cannot choose between these!
6. My Love – Lee Hi (Moon Lovers)
7. Say It! What R U Doing? – K. Will (DOTS)
8. After A Long Time –  Baek Ji Young (Rooftop Prince)
9. I’m Going to Meet You – Deok Hwan (Queen Inhyun’s Man)
10. Falling Tears  – Shin Jae (49 Days)
Carry On - ALi (Faith)
Reset - Tiger JK (Who Are You - School 2015)
I'll Listen To Your Story - t Yoon MiRae (Who Are You - School 2015)
Such - Kang Hyun Min ft. Jo Hyun Ah (Cheese In The Trap)
I Am Love - Tearliner ft. Yozo (Cheese In The Trap)
A Little Braver - New Empire (Uncontrollably Fond)
Seredipity - 2Young (The Heirs)
As Time Goes By - Ki Hyun (Reply 1988)
Be With You - AKMU (Moon Lovers)
Wind - Jung Seunghwan (Moon Lovers)
You - Ben (Healer)
Perhaps Love - J&Howl (Goong)
With You...I'm A Fool - Stay (Goong)
Love Rain - Jang Geun Seuk (Love Rain)
Love Is Like Rain - Na Yoon Kwon (Love Rain)
Echo - Every Single Day (I Hear Your Voice)
That Love - Jung Yup (Doctors)
Everytime - ChenXPunch (Descendants of The Sun)
Too Good - Junggigo ft. Minwoo of Boyfriend (High School Love On)
Stand By Me - SHINee (Boys Over Flowers)
Lucky - Ashily (Boys Over Flowers)
Fight The Bad Feeling - TMax (Boys Over Flowers)
Do You Know - Someday (Boys Over Flowers)
One More Time - Tree Bicycle(Boys Over Flowers) 
Like A Fool - 2AM (Personal Taste)
She is - Clazziquai Project (My Lovely Sam Soon)
Confession - Delispice (Reply 1997)
Uneasy Romance - Echae En Route (Oh Hae Young Again)
C'mon Through - Lasse Lindh (Soulmate)
Could Give You Love - Lasse Lindh (I Need Romance 2)
Goodbye - The Melody (Coffee Prince)
Emergency Room - Izi
Don't Forget - Baek Jiyoung (Iris)
Even If I Die, It's You - BTS V and Jin (Hwarang)
Auditory Hallucinations - Jang Jae In (Kill Me Heal Me)
My Destiny - Lyn (My Love From The Star)
Love is the Moment - Homme (Heirs)
Confession - Deli Spice (Reply 1997)
Aaaah, some of these songs bring back so much memories! I love listening to each of them. :)
For me mostly from hwarang and goblin ost. I'm so in love with their OST's.
HWARANG:::
Evwn if I die, it is you- v and jin
The devine move- yang yoseob
Our tears- Hyolyn
Only you- Wendy
Dream- bolbbalgan4

GOBLIN:::
My eyes -10cm
Beautiful-crush
Stay with me-Chanyeol :)

SPECIAL MENTION:
Confession-DELISPICE (Reply 1997)- I thought I was the only one who likes it cause it sounds like an old music :)
To you- Sung Si Kyung ( Reply 1994)
Go- SALTNPAPER

Shiny Day - Lily M, JACE
Moonlight is Setting - Heora
All For You - Seo Inguk, Jung Eunji
Auditory Hallucinations - Jang Jae In
Reset - Tiger JK
Touch Love - Yoon Mirae
Couple - Sechskies

The full DoTS, Goblin, and Another Miss Oh osts, +
Isn't She Lovely - Park Boram (Introverted Boss)
Reset - Tiger JK (School 2015)
The Tiger Moth - Monsta X (Shopping King Louie)
Auditory Hallucinatons - Jang Jae In (Kill Me, Heal Me)
Don't Push Me - Wendy, Seulgi (Uncontrollably Fond)
Where Are U - Jung Joon Young (W)
I'll Pick You Up - Standing Egg (WLFKBJ)
you are my everything---gummy
this love--davichi
you--ben{healer}
moonlight--gummy{love in the moonlight}
unstoppable sun--aron yan[just you]
There are so many, and I'm sure people will list from years back but I want to give a mention to the most recent OST that I downloaded in complete:

On the Way to the Airport OST