Hidden gems/Unknown dramas?

I would recommend:

• Circle 
• Long Vacation 
• Nigeru wa Haji da ga Yaku ni Tatsu 
• Pride 
• Tunnel 
• The Guest 
• Boku dake ga Inai Machi 
• Girls' Generation 1979 
• Tientsin Mystic 
• Todome no Kiss 
• Toki wo Kakeru Shoujo 
• BORDER 
• dele 

Most of these are J-dramas (they usually have less watchers in general on MDL compared to Kdramas). Hope you find something you enjoy! :)

Karamazov no Kyodai

Yakou Kanransha

N no Tame ni

Long Love Letter

Manhattan Love Story

Biblia koshodou no Jiken Techou

Evasive Inquiry Agency

Most of them have under 1000 ratings but I really enjoyed them. 

imo I don't really consider The Guest, Tunnel, Circle to be "unknown dramas" 

Priest is more of an underrated drama for me since it has less watchers, the energumen have supernatural powers compared to The Guest where they just go around killing people with knives?

Spring Turns to Spring hilarious drama, no romance

Woman of Dignity I love the FLs in this. I shipped Kim Hee Sun with Lee Ki Woo <3 this drama has only 4312 watchers compared to SKY Castle & The World of the Married 

The Promise this was the last daily drama I completed - it's good if you're into makjang dramas!

A Pledge to God the style of this drama resembles a daily drama but with shorter episodes, this has only 997 watchers!

Love in Sadness (remake of Utsukushii Hito) if anyone likes watching obsessive/stalker guys this drama is for you xD

Mistress mystery/thriller

Children of Nobody 

Perfume rom com

The Package rom com, nice sceneries of France

Legal High remake of Legal High j-drama. Jin Goo was great in this xD

Love Affairs in the Afternoon remake of Hirugao

My Love Eun Dong 

 Sunset:

imo I don't really consider The Guest, Tunnel, Circle to be "unknown dramas" 

I completely agree but unfortunately of the K-dramas I've seen those were the only less known/hidden gem ones I could think of :(

 Wintergardener :

• A Piece of Your Mind
• Come and Hug Me 
• 365: Repeat the Year

Piece of your mind +++++++++100000000 for this masterpiece. It's so so underrated that many people pass by this amazing drama, and some who watch it don't appreciate it as much. This drama is not for everyone, only ones with depth of mind can understand the beauty of this artistic piece.

Stranger/Secret Forest - season 2 coming soon! so twisty-turny that it's hard to binge-watch IMO.  I think it's on US netflix right now, About 8K watchers.

The Good Wife  - I really enjoyed the Korean version (haven't seen the US version).  (4700 watchers)

Heart to Heart - quirky characters, unique love story, touches on mental illness & recovery, weird hidden identity (5000 watchers)

Kimchee Family/Fermentation Family - wonderful little family drama that combines food, romance, and mystery (this one only has about 1800 watchers). I watched this one with a non-Kdrama fan & he enjoyed it (especially the food scenes)  

Not entirely unknown, but people either seem to have forgot about it overnight or just never liked it much in the first place - She Is Beautiful, a 2018 Chinese drama that I loved. I have never heard or seen anyone mention it, which is incredible to me because it’s so enjoyable and cute. Whilst I believe it’s rated quite well, or watched by quite some users on here - it seems to have been very underrated which is really sad because it’s so good to me. 

 Margaret:

Stranger/Secret Forest - season 2 coming soon! so twisty-turny that it's hard to binge-watch IMO.  I think it's on US netflix right now, About 8K watchers.

The Good Wife  - I really enjoyed the Korean version (haven't seen the US version).  (4700 watchers)

Heart to Heart - quirky characters, unique love story, touches on mental illness & recovery, weird hidden identity (5000 watchers)

Kimchee Family/Fermentation Family - wonderful little family drama that combines food, romance, and mystery (this one only has about 1800 watchers). I watched this one with a non-Kdrama fan & he enjoyed it (especially the food scenes)  

Watched Heart to Heart because of your suggestion and WOW. What a fabulous drama!!! Thank You!