Heirs
She's so Lovable
Boys over Flowers

Have to add another to my list, Kill Me, Heal Me. It's okay but it's not as great as people seem to claim.
My Love from another Star: really boring and the main couple didn't convince me. It's for me the most overrated drama ever.
Healer
Secret Garden
You're Beautiful
Boys Over Flowers- Hate the "women want men who treat them badly" trope.
You're Beautiful - Meh.
The Girl Who Sees Scents - Female lead annoyed me to death.
It's Okay, That's Love. 
FULL HOUSE (Korean). Everyone talked about it how good it was, I watched it, but for me it was just OK. I did not get it. It was all screams and chaos. I preferred Full House 2. The best version was THAI one, which I watched it recently. That one was spectacular! 

I HEAR YOUR VOICE was another. As most of the people loved it, for me it was borrring!!! I did not get any of it, did not like it, tried to watch it twice and quit by mid of it. I don't know but this one never did anything to me and all world buzz about it ... still don't understand why

I NEED ROMANCE 3 it was just fine to me but this one was dull for me at the end. Honestly that one did not have to exist for me. Nothing against noona romances, but the lead just got too loud for my ears. I don't like overreacting women!

HANA KIMI although it was funny, it was just too silly for me. I found it childish. Honestly I did not laugh at all. I also skip this drama from mid to end.
Definitely Boys over Flowers and also I would have dropped You're Beautiful if Jeremy hadn't been in it. 
^^ Same here with You're Beautiful. Jeremy was the only reason I kept watching. Loved him. Shinwoo was okay too.
Heirs
Hana Kimi
Healer
Secret Garden [Most boring drama in my opinion]
Kill Me Heal me
Innocent Man
That Winter, The wind blows
You who came from the Stars
You're Beautiful
Secret Garden
Kill me, heal me, Bad guys, Incomplete life. All top rated and i couldn't finish any of them. Perhaps this is because they are still new and ratings have not settled yet. From earlier period: City hunter, Nine: Nine times time travel (barely watchable), a Gentleman's dignity..... and also almost every drama that contains word "flower" in its title is greatly overrated. 
I have a whole 18 of them if not more...
Overrated Dramas
Dream High (hated the main characters sooo much. Jason and Pil Suk made the show for me)
Heirs (hated the plot & acting wise; park shin hye plays the same roll all her dramas except for FBND which happens to be one of my favs)
Kill Me, Heal Me (don't even get me started)
You who came from the Stars (i just wasn't a fan of the characters but nice plot. I love Kim Soo Hyun to bits but i hate like 85% of all his roles in dramas)
She's so Lovable (zzzzzzz)
Masters Sun (i didn't like the acting but it wasn't the worst. Gong Hyo Jin is still one of my favorites Actresses though)
To this day I have repeatedly tried watching Bridal Mask and City Hunter to no avail. I always lose interest right around episode 6. Maybe one day.....

And I wasn't a huge fan of Kill Me, Heal Me. Unfortunately, I'm just not that big of a fan of either of the lead actors...which is probably why Secret is still on the 'indefinite' Plan to Watch list.