I haven't been watching Kdramas as much as I would like T_T But my favorites are Reply 1997, Love Rain, Heartstrings, Gu Family Book, and Let's Eat. I also like What's Up, Blood, and Trot Lovers but the ends aren't that good. I agree with the people below. My love from another star and Monstar :)

you might've watched these already but
Pinocchio, God's Gift 14 Days, Remember - Son's War, My Love From Another Star

everyone needs to watch KILL ME HEAL ME.
its a must.
seriously

I'm watching Remember with Yoo Seungho in it and it's pretty amazing so far. I also recommend Healer as well.

I liked a lot of older ones: secret garden, dream high, monstar (but the ending isn't so good, not bad, just not what you'd expect)