only the classics like nobuta wo produce, 1 litre of tears, hanakimi, hana yori dango/bof, my boss my hero & atashinchi no danshi because these are the gems that got me into dramas so i like to revisit them from time to time.
Reply 1988
Princess Hours
Full House
Goblin
Silence
1 Litre of Tears
Boys Before Flowers
Sassy Girl Chun Hyang
Moon Lovers: Scarlet Heart Ryeo
The Princess' Man
Rooftop Prince
Doctor Stranger
Emergency Couple
Sungkyunkwan Scandal
The moon that embraces the sun
Playful kiss ALL VERSIONS
eternal love
you who came from another star
descendant of the sun
Not much of a rewatcher also, but I've watched Goblin so many times. I think I've watched every episode a minimum of 3 times. Other than that I've rewatched Nobunaga Concerto and started to but didn't finish rewatching Pinocchio.
Eternal Love
You Who Came from the Stars
My Girlfriend is a Gumiho
Sungkyunkwan Scandal
The Master's Sun
Rooftop Prince
Pinocchio
secret garden
secret
a gentleman's dignity
secret
innocent man
fated to love you
descendants of the sun
oh my venus
 
the only drama ive been able to rewatch completely is my amazing boyfriend. ive rewatched a few episodes of city hunter, healer and IRIS 2
I rewatch dramas frequently, this are the ones that i rewatched full and in order, but if i considered the times i watch just random parts the list would be much bigger.

5+ times : 
Reply 1997 (2012)

2-3 times:
Empress Ki (2013)
Prime Minister and I (2013)
Six Flying Dragons (2015)
You Who Came from the Stars (2013)
Deep Rooted Tree (2011)

At least 1:
Another Miss Oh (2016)
Drinking Solo (2016)
Itazura na Kiss - Love in Tokyo (2013)
Moon Lovers: Scarlet Heart Ryeo (2016)
My Wife’s Having an Affair this Week (2016)
Secret Garden (2010)
The Heirs (2013)
The Producers (2015)
Uncontrollably Fond (2016)
You're Beautiful (2009)
Kill me, Heal me
The Master's sun
The Princess' Man 
Hana Yori Dango
My Girlfriend is a gumiho

I'm honestly not much of a re-watcher, since I prefer to discover new dramas haha
A Love To Kill
Another Miss Oh
Arang and the Magistrate
Boys Over Flowers
Chief Kim
Cruel City
Fated to Love You (Korea)
Fated to Love You (Taiwan)
Flower Boy Ramen Shop
Goblin
Goong
Healer
I Need Romance 3
I Remember You
Innocent Man
It Started with a Kiss
It's Okay, That's Love
Kiss Me (Thailand)
Kleun Cheewit (Thailand)
Kou Kou Kyoushi 2003
Legend of the Blue Sea
Lucky Romance
Marriage, Not Dating
My Girlfriend is a Gumiho
Oh My Venus
Queen In Hyun's Man
Reply 1988
Strong Woman Do Bong Soon
The Devil
They Kiss Again
Tomorrow With You
Weightlifting Fairy Kim Bok Joo
You Who Came from the Stars
My Amazing Boyfriend
Strong Woman Do Boon Soon
Full House (Thailand)
Fall In Love With Me
Love Around
Love Now
Murphy's Law of Love
I rarely re-watch dramas...

Nirvana in Fire
Healer
Empress Ki
Goong
Boys over Flowers