I depends on your taste but -
Heard it through the grapevine. I just can't believe the rating for this, it should at least be an 8.5. Great comedy, great family drama, great story, brilliant script, great visuals and great acting. You'll be hard pressed to find another Kdrama like it.
Cross. One of my favourite revenge and redemption stories. It's quite probable this one is rated so low because of the scandal created by the actor for the mentor character while it was airing. This resulted in a completely different ending, but the writers handled it well, very well.
Solomon's Perjury. My favourite school drama, with believably intelligent, diverse characters and a great mystery. Don't let the premise fool you, it hardly feels like a school drama. Also good performances from such a young cast.
Save Me 2. Despite being very different and nearly unrelated, it should be rated equally with the first one. I enjoyed it more, and if you can handle the slow build up, you will find it oh so worth it as well.
All these have a low rating, but I consider them hidden gems.
Also I second Just Between Lovers and Miss Hammurabi, but I won't call them particularly underrated.