I'm a huge lover of OSTs! Here's some dramas that I would recommend:
Kill It - Not really much romance but great plot and I personally love the OST. I suggest you listen to Like, You, Me by Kim Bo Kyung or Forever Love by Min Kyung Hoon
The Princess Wei Young
Someday or One Day - !!!. Slight love triangle. I suggest you listen to Last Dance by Wu Bai (The main song in the drama that is very important to the plot) or Someday or One Day by Shi Shi (The opening song).
The Love Equations
Goblin - Its been over a year since I watched this drama and a lot of songs from the OST have stuck with me :). No love triangle. I suggest you listen to Hush by Lasse Lindh or I Will Go to You Like The First Snow by Ailee
Story of Yanxi Palace
The Eternal Love - Love Love or 学到老爱到老 by Jin Run Ji!
Descendants of the Sun - Super super pretty OST, I quite like the instrumentals.No love triangle. I suggest you listen to You Are My Everything by Gummy or Always by Yoon Mirae
Find Yourself - No love triangle. I suggest you listen to Amazing by Lin Shi Min or 风的来信 (Feng De Lai Xin) byJin Wen Qi
Moonshine and Valentine - No love triangle
Love in Sadness - Has a love triangle but if you/re looking for a drama with a great and emotional soundtrack I suggest this drama. Super underrated drama. I suggest you listen to Stay With Me by Cheon Dan Bi or Loving You Everyways by Kang Hye Jung.
The Romance of Tiger and Rose - All songs in this OST are honestly quite good
Love Affairs in the Afternoon - The songs are instrumentals in this drama are truly beautiful, I suggest you listen to One by Woong San or Bubble by Jang Hea Jin.