I loved Shin Sung Rok since YWCFTS, Lee El since Liar Game, and Ji Sung since KMHM.
There have been other dramas that put a certain actor or actress on my radar, like Jun Ji Hyun in YWCFTS, Yook Sungjae in Who Are You - School 2015, and many many other I want to see more of now.
On the other hand, the opposite has sometimes happened too. I loved Yamapi in Nobuta wo Produce but not in anything I've seen him in after that.