Lee Bo Young
Kim Go Eun
Kim Jo Won

As for your second question-I think script matters a lot.But sometimes even good content fails to connect with the audience without good actors.Sometimes good actors can make even the average content look good/satisfying to watch.But I'll say 60-40 for content/acting I guess.The show which wins most hearts and is cherished for a long time will ultimately be one which has a combination of both.IMO one which lacks in either department might be entertaining for one watch but won't leave you wishing for more or won't make you wanna marathon it again.

I know many people probably would disagree but I think Hwang Jung Eum's acting is really genius

1- Kim Seul-gi

2-Jun Ji Hyun

3-Son Ye-jin 

4- Seo  Hyun Jin

5- Han Ji-min

(These are my opinions, I know that everyone won't agree) ((The actresses are in order, like them just the same))

Gong Hyo Jin - She is the epitome of "independant" female roles in Korean dramas so of course I love her. I feel that she is a bit type casted and that she is in a bit of a slump but her work in the past has been interesting and entertaining! Feels like I don't have to say that much about her, everyone knows who she is and how good movies and dramas she has been a part of. 

Seo Ye Ji - Great actress that has showed big improvements! Seo Ye Ji had a bit of a rough start with Moorim School and Potato Star 2017QR3, but I don't feel like it would be fair to blame that on her (the script and concept sucked). She is great with dramatical roles that has some dark plots, she can really stir up emotions in the viewer and stands out in the crowd of bleak actresses. She acted perfectly in the thriller drama Save Me, which is still her best work til date. I wish that she would get more A-list roles, maybe on the next drama by Kim Won Suk (director of Signal and My Mister). 

Lee Ji Eun - Also known as IU. Some people won't agree with me on this (trust me I understand that BUT listen to what I have to say). She has done some "bleak and boring" performances in the start of her acting career. She felt a bit distant from the viewer and kind of awkward? But I feel that her role in My Mister alone is enough to prove of her talents in acting. I feel that she will only get better over time because I feel that she just has something that makes people focus on her even though isn't the most expressive actress she delivers one of the (maybe the best?) female acting performance in a drama 2018. 

Kim Go Eun - Horror/Thriller queen. I feel that she really hasn't done any bad work, maybe Cheese in the Trap but I feel that was the scriptwriters fault. From a gripping debut in the melodrama A Muse to big hit movies like Monstar, Coin Locker Girl and popular drama Goblin; Kim Go Eun delivers emotional roles no matter what genre she is in. I feel that her innocent and "ordinary" looks makes her a even better actress who can transform into whatever she wants. 

Kim Hye Soo - One of my favorites of all time, she can play villains, heroes, the romantic interest and the opposing mother. Her acting career has been filled to the brim with good movies and dramas that shows her ability to portrait anyone she wants. Diverse with a great technique makes you remember her performances for a long time, this is especially prominent in Coin Locker Girl where she acts as the loan shark "Mother". 

I would list few of them that I remember at most:

Song Ji Hyo and Gong Hyo Jin are my favorite among older actresses and as far as younger actresses are concerned Lee Seung Kyung, Kim Tae Ri, Im Ji Young and Park Bo Young  tend to show off talent as well as quality as far as acting is concerned and it is almost never cringy to watch them perform (depending on the role tho). Among male population, I would say that Ji Sung, Jo In Sung, Nam Goong Min and Lee Kwang Soo fit all genre they appear in as well as Jang Hyuk who really fits in both comedy and action. I would really love if Lee Kwang Soo was given more opportunity to occupy more space in dramas  as I think that he is usually put as a support role due to higher popularity of main leads who tend to be regarded as better looking and more handsome. 


Also, lately Jung Ryeo Won caught my attention ;D  I would hope to see more dramas with her as a lead too. 

song hye kyo

kim hee sun

Son Ye-jin

Ryu Hwa Young I've seen her in the drama Mad Dog and she's wonderful. She even won the Best New Actress Award in 2017.