Before anything, I have to say that I tend to rate high dramas with unusual themes.
So now, here are some (in my opinion) underated dramas :
When it's at night (my rating : 9/10 vs community rating : 7.1/10)
=>romantic police drama at the Cultural Theft Devision
Spotlight (my rating : 9/10 vs community rating : 6.4/10)
=> about reporters, I don't remember any romance in this one though
Who are you (my rating : 10/10 vs community rating : 7.6/10)
=>a policewoman can see ghost after being in a coma for 6 years + a bit of noona romance in this one. It still has an alright rating but it was one of my favourites so 7.6 fells a bit low.
One missed call (my rating : 10/10 vs community rating : 6.8/10)
=> police/horror drama, just don't expect blood and you'll appreciate it.
Operation proposal (my rating : 9/10 vs community rating : 7.3/10)
=> personally, I prefered this korean version.
Painter of the wind (my rating : 10/10 vs community rating : 7.9/10)
=> Okay, this drama is a MASTERPIECE, even more so if you like gender-bender theme. I don't even understand why the rating dropped so much.