My (biased, uneven) rating scale:

10.0 - Absolute masterpiece. Enjoyed everything in the show, well-paced from the start to the end, e.g. Amachan, Kill Me Heal Me. Also reserved for my ultimate favourites (where I ignore any glaring plotholes because I'm biased) e.g. The Greatest Love, Fated To Love You, Trick

9.5 - Really well-made, loved everything about it except for few minor details e.g When The Camellia Blooms, Flower of Evil, Jealousy Incarnate

9.0 - Loved it for majority of its run, but some plot holes were too glaring to be ignored or certain aspects went in a direction I disliked e.g Oh My Baby, Quartet, Remember

8.5 - Loved it overall, but show lacked in some aspects like a bad ending, slow pacing in the middle or got extended unknowingly and dragged on e.g. Mirror of The Witch, Memorist, Missing 9 

8.0 - Liked it, quite a good production overall but show had too many things I disliked e.g. Doctor John, Keizoku, I Hear Your Voice. 

7.5 - Show had potential but faltered along the way, and just not entertaining enough. A meh drama e.g. The Fiery Priest, Secret Garden, Warm and Cozy

7.0 - Show had a good premise but failed in its execution e.g. Mr Baek, Train, Plus Nine Boys

6.5 - Got bored after a while or had too many illogical plots or characters e.g. Alice, Itaewon Class, Shine or Go Crazy

6.0 - Show was even more boring and more illogical e.g. Birth of A Beauty, Cheongdamdong Alice, Wok of Love

Below 6.0 - Really not worth your time, please skip it, it's just bad