I'll always miss Coffee Prince I guess, it was my first kdrama and it's still my favorite, it had the full package, good writing, perfect cast, great OST.
Very often I catch myself missing 49 Days, it was so unique, where can I find something similar? Good drama with a touch of fantasy, good cast and again I really liked the OST, every ep had a cliffhanger and I did like the ending!
The third is You're Beautiful, I was angry for a week after it ended, I'm still missing it! :P