If I had to choose my absolute favorite drama, I would need to probably choose Moon Lovers: Scarlet Heart Ryero. The cinematography was amazing, the reverse harem was done masterfully and nothing dragged. I loved the characters, the story and the OST. The chemistry was also great.
If I had to choose my top 3, I would add Healer (I still don't know if Moon Lovers or Healer are my definite number 1) and Another Miss Oh. Healer had great leads with sizzling chemistry, engaging story and a really kick ass male lead. Another Miss Oh was one of the most intriguing stories I've seen. It also has one of the best kiss scenes.
If I had to do my top 5, I would add Because This is my First Life and Kill Me, Heal Me. BTsMFL is very refreshing in how non-cliche the dialogues and the relationships are. I loved everything apart from the last episode. Kill Me, Heal Me is amazing because of Ji Sung.
If we don't put the number on the best shows I've ever seen, I would add some more. Haven't seen My Mister (plan to do it this weekend, actually) or Save Me, but I definitely agree with melondrx's Go Back Couple and Age of Youth. Personally, I also loved Falling for Innocence and The King's Woman.