Drama Rating I think, is very subjective to our personal experiences and mood at the time we watch the drama. There are no Objective way to rate a drama. We can choose categories, such as plot, chemistry between the actors, best picture, or best OST,  but, when it comes to a drama we like, it's how it move our emotions. Did we laugh, cry, was surprise, bored or felt nothing at all, our emotions at the time we watched a drama dictates the rating.

For me personally, if I can watch a drama again and again and still love it, then I would give it a higher rating, than a drama that can only hold my interest for a short period of time. That said, Masters Sun, it's Okay that's love, boys over flower and heirs are dramas I can watch again and again, while healer and You who came from the stars, I can not finish it the first time around. 

My two cents
Overrated: Mary Stayed Out Last Night and Gong 

Underrated: Biscuit Teacher and Star Candy, Scent of a Woman, Personal Taste, Noble, My Love and Thank You. But I wish others would give Cdramas more of a try too. They often drag less than many of the kdramas I've seen and have more content so you can really sink your teeth into a drama. Boss & Me, Scarlet Heart and and Cruel Romance are some of my favs.
Over-rated - 100% HEIRS, High School King of Savvy, Reply 1997, MARS.

Under-rated - Heart to Heart, Smile, you, White Christmas,
Over rated: Oh my Ghost and Pinocchio...I think I'm the only person who didn't really like LJS and PSH

Under rated: Ho Goo's Love (Fool's Love) 
overrated- anything with Park Yoo Chun in it and Heirs...ugh that was awful.. and i need romance 1,2 and 3.

underrated-  Valid love, yes it's got great reviews, but it was only rated by 790 users, compared to over 6,000 for marriage not dating.
Oh, this is fun.  I stole and modified @ViolinGal's method of comparing my scores to the overall community's because I'm a nerd and that's fun for me.  My score's deviation from the average in bold.

So in terms of score only, I guess the dramas I think are the most overrated are:
1) Warrior Baek Dong Soo (so much promise, so little payoff.  And such a worthless ending.) -3.8
2) I Really Really Like You (cute at times, but so, so draggy and so little plot) -3.3
3) Prince's Prince (okay for what it was, but I'm unwilling to rate such a messy script any higher than I did) -2.7
4) Coffee Prince (just didn't really care for it, it was just okay for me) -2.4

In terms of score for underrated dramas:
1) A Man's Story (love this drama, does no one else?  lol) 1.5
2) Tomorrow's Cantabile (enjoyed it just as much as the j-drama and liked them both a lot) 1.3
3) Gaksitaaaaal/Bridal Mask (only drama I have ever given a 10) 1.2
4) Flowers for My Life (probably fits every possible definition of underrated IMO) 1.2

But in terms of how much buzz a drama has, using how many people have rated it as a rough measure, there are a couple more on my list that I feel are really excellent dramas that would be considered underrated by that definition.  I'd put both Missing Noir M and Once Upon a Time in Saengchori (only 367 people!) in that category.  Flowers for My Life and A Man's Story both have very few ratings as well as lower scores than I think they deserve (ha), and since I love them both dearly, they probably tie for my overall most underrated drama.

There are plenty of other dramas that I could probably call overrated, but I can't seem to finish most dramas that I truly dislike, and I won't rate anything I don't finish.  So my list is a little skewed in that sense.
For overrated dramas (based on buzz and critical reviews) the first three that came to mind are: Coffee Prince, You're Beautiful, and City Hall. Not that I didn't enjoy those dramas, I did, but not as much as the rest of the drama-watching world apparently. They're all at least 6 years old and yet are still consistently on people's best dramas list (proving their staying power) but I found it hard to really get invested in them (they also all have at least a 7.9 rating by at least a couple thousand reviewers). 

For underrated dramas, the first couple that come to my mind are Surgeon Bong Dal Hee and H.I.T, I love those dramas and they do have fairly good reviews, but hardly any buzz (less than 300 ratings on MDL). I also agree with Flowers for My Life, and for another investigative one I would say Sign as well (it's a 2011 drama but has less than 500 ratings, though the existing ratings are fairly high). 

I realize most of the ones I've listed are older dramas, and I can think of more recent overrated/underrated dramas as well, but I think it's easier to be objective after a few years have gone by and you're less influenced by the hype (and several dramas people have listed as underrated I would say don't quite fit that category if you're judging by the amount of times they pop up on people's "must see dramas" lists- but that of course depends on your criteria).
Just first few that come to mind now for overrated: Doctor stranger, You who came from stars, Healer, Secret, Secret garden, Girl who sees scents, Boys over flower, Playful kiss, Kill me heal me, Sassy Go Go, etc.

Almost don't see anything Underrated except: My Lovely Girl (2014) (really underrated!), Flower Boy Next Door, The Producers and Voice (2009).
Overrarted: Kill me heal me, The master's sun, Secret garden, You're Beautiful (with Park Shin Hye), King of Baking Kim Tak Goo. I  liked the first one but it doesnt deserve a 9, the last three have a bad story, and you're beautiful a terrible acting.

Underrated: Surplus princess, Ex-Girlfriend Club, Mary Stayed out all night, Once Upon a Time in Saengchori, Angry mom.
Overrated: The Heirs! Lee Min Ho's character in this drama... I've never hated someone so much in my life.
Underrated: Heard it Through the Grapevine! A 7.7 rating? wat
Underrated: A Man's Story
Overrated: The Master's Sun
I think bad guys is quite underrated and The Heirs overrated. 
Overrated: You Who Came from the Stars, It's okay, that's love, Replay 1997, I need Romance

Can't think of anything really underrated right now. 
Overrated: I Remember You, it's not a bad drama but I feel like it was hyped so much that I was let down when I watched it (and Jang Nara and Seo In Guk had no chemistry together, making it kind of painful to watch)
Underrated: Ho Goo's Love, funny and addicting, I'm surprised it's not more popular!
Overrated: Heirs, Kill Me Heal Me, I Remember You, Reply 1997

Underrated: Reply 1994, Tomorrow's Cantabile, Heart to Heart, D-Day