Overrated: Boys Over Flowers, Oh Hae Young Again, and Coffee Prince.

Underrated: Incarnation of Money and Misaeng (people assume that it's a boring drama, but in reality it's a really touching drama).
Underrated: Ningen Shikkaku ~ Tatoeba Boku ga Shindara, Ikebukuro West Gate Park, LIFE, The Quiz Show

Overrated: Coffee Prince, Boys Over Flowers (all versions), Heirs, Secret Garden
Underrated: the Legend (2007), first supernatural saeguk I've ever watched. It is so epic!! and the Three Musketeers (2014)

Overrated:Boys Over Flowers, You're beautiful, You who came from the stars, the Heirs, etc...
overrated:coffee prince, empress ki
underrated: orange marmalade, shut up flower boy band, it's okay it's love
Underrated: signal- this is a freaking awesome drama! 
overrated: the heirs 
Overrated : Definitely DOTS (descendants of the sun) 

Underrated: To the beautiful you korean version. 
Underrated - Wife's Credentials / Lie to Me / Secret / Can you hear my heart? / Heart to Heart / Scent of A woman / That Winter The Wind Blows / Hotel King / Bad Love / Marriage Not Dating / I Remember You / I Miss You / A Gentleman's Dignity / What Happens To My Family / Mask / Smile You / Five Children / Heart to Heart / Boss & Me / Bad Guy / Stars Falling From The Sky / Prosecutor Princess / The Innocent Man / Scent of a Woman / 

Overrated - Heirs / Pinocchio / I hear your voice / I Need Romance 3 / Full House / Madame Antoine / It's OK, That's Love / RoofTop Prince / Fated To Love  You / My Name is Kim Sam Soon / Kill Me Heal Me / Sassy Go Go / Reply (all) / We Are All Surrounded / Imaginary Cat / The Greatest Love / Modern Farmer /
Overrated: there's a lot but Healer's rating is just ridiculous... also, I know it's not a drama but A Werewolf Boy is extremely overrated too
Underrated: Jackpot- I just completed it and it was really enjoyable even though I'm not a fan of historical dramas
Overrated: Flower Boy Ramyun Shop, Queen Inhyun's Man (I have these two dramas since forever on hold, somehow I couldn't get past the first episodes), Heirs,
Underrated: God's Quiz 1-4, Missing Noir M, Cruel City, White Christmas, Shoot My Heart, Angry Mom, Incarnation of Money, She is Wow
Overrated : Descendants of the Sun, It's Okay Its love, Sassy Go Go, Secret Garden
Underrated: Angry Mom, Signal ( One of my favourite korean drama)
underrated: angrymom, heart to heart, marriage not dating,  
overrated: descendant of the sun, yo who came from the stars, doctor stranger
Going by MDL ratings and rankings..

Underrated: Flowers for My Life (very!), Heart to Heart (maybe, still rated pretty well), Miss Korea, Dal Ja's Spring, Who Are You? (2008 one), Soulmate (somewhat)

Overrated: Secret Garden (ridiculous), Kill Me Heal Me (ditto)

Thought about listing those I found overrated and still liked to a degree, but that would have taken a while, and I didn't want to be too negative.
Overrated - Heirs and Descendants of the Sun (by a mile)
Underrated - Vineyard Man, Bad Couple, Lawyers of Korea, My Sweet City
Overrated: Heirs, Descendants of the Sun, Another Miss Oh, You Who Came from the Stars.
Underrated : Misaeng, Brain, Ms. Temper & Nam Jung Gi, Secret, That Winter The Wind Blows, Angry Mom. 
Descendants of the Sun ... just annoying that everyone likes it cause it was not even a real drama for me, it had no story 
Mrs Temper and Nam Jung Gi <3 just the best drama ever