a poem a day (relatively underrated) - it's not an intense plot so it's an easy watch, it's on the slow side but i really enjoyed watching it. it's a realictic drama but also very fluffy. i really like the romance, it's one of my faves in any drama cause it's not toxic one bit it's just cute. also all the characters are very likable.
just dance (underrated) - this one is very underrated compared to how amazing it is in my opinion. the lead female is a bit of a anti-hero it portrays how there is a very thin line between going after what you want and hurting people in the process. it also portrays the difficulties the characters go through very beutifully. but it also has fun moments and building friendships.
last minute romance (i guess underrated) - this one is a very short drama with 2 episodes but i really liked it. it's very basic and you know what's gonna happen from the start but it didn't stop me from crying and i really like the message it had at the end.
detectives of seonam girl's high (ahead of it's time) - this one is a bit old and was very controversial at the time cause it had some sensitive topics like lgbt and suicide and teen pregnancy but i really i love the they portrayed these issues, not positively or negatively but objectively.
these are the ones i've watched, i guess they're not that underrated but they're not very popular either. i hope this helps and you found find something that fits your taste.