I don't think many will agree with me on these but Overrated:
Goong
Full House
My lovely Kim Sam Soon

Underrated-
Not sure, nothing realy
I'll go with Suzuki Sensei and Arakawa Under the Bridge as very underrated.

For overrated I'd say Great Teacher Onizuka (1998) and Gokusen. Don't get me wrong, they are very good shows, but not the best dramas ever which they often rate as basically. At least they don't hold up as that these days.
Underrated: Time Slip Dr. Jin Tree of Heaven K POP The Ultimate Audition Let's go the beach Panda and Hedgehog Overrated: Flower Boy Ramyun Shop Coffee Prince What's Up
Underrated: It's Okay, Daddy's Girl (this another best Moon Chae-Won drama) Panda and Hedgehog Miss Ripley (Why low score in MDL? This drama very good) Dummy mommy Lights and Shadows Five Fingers Overrated: Shut Up: Flower Boy Band (this very boring drama for me) Faith Arang and the magistrate My Girlfriend is a Gumiho Hong Gil Dong Iljimae (The Return of Iljimae better than this)
Over-Rated: Any drama that is on my dropped list that is/was popular. Under-Rated, there are a lot so I will only name 2: The Mighty Chil Woo or Strongest Chil Woo Akutou ~ Juuhanzai Sousahan
Under-rated:
15 Sai no Blues
FACE MAKER
Bloody Monday


Over-rated:
Gokusen (Bored after watching two episodes)
Dream High
Hana Kimi
Sang Doo Let's Go To School -> The most underrated drama ever. Queen In Hyun's Man -> The most overrated drama ever
Underrated: Prosecutor Princess, Take Care of the Young Lady, BIG, Gakeppuchi no Erii, Holyland Overrated: Heartstrings, Shut Up Flower Boy Band
Underrated: Kimi wa Petto Overrated: Goong
Underrated:
Sweet 18
Down With Love
Smiling Pasta

Overrated:
Playful Kiss
In Time With You
Heartstrings
Overrated: Reply 1997 Underrated: White Christmas and Time Between Dog and Wolf (seriously, if you haven't seen these two dramas, you must! They're simply awesome! <3
Underrated: Padam Padam I Love Lee Tae Ri Overrated: Rooftop Prince Faith Missing You
Some of the dramas people have listed as overrated makes me sad :(

Underrated
KPOP The Ultimate Audition (Don't get me wrong, this is by no means a fantastic drama, but I think it gets a lot of crap from people, while still being enjoyable enough to watch.)
Haeundae Lovers (This drama was seriously fun to watch, even if it isn't the best or most believable.)


Overrated
Flower Boy Ramyun Shop
Playful Kiss