Things from my list that I have rated at least 8.0, and I have rated them higher than their average score (so all of them are 7.9 or less):
1. Flowers for My Life
2. Tomorrow's Cantabile (this will depend on how attached you are to the j-version and/or manga; I actually like all versions I've encountered, including the k-version!)
3. Taereung National Village
4. Babysitter
5. Once Upon a Time in Saengchori
6. Full House Take 2 (this will depend on how distracted you are by terrible hair.)
Listed from highest to lowest score according to me :) And it's pretty rare that my score is actually higher than the average.
There is a giant thread of underrated dramas somewhere, but I'm too lazy to look for it right now. You might find some ideas there too.
ETA: I think
this is the thread I was thinking of. Happy hunting!