1. Kim Yoo Jung--I saw her in Moon Embracing the Sun and I couldn't believe that was a 12 year old. IMO, the scene where she accepts the poison and dies in her father's arms is one of the great performances in cinema history. Certainly the greatest performance by a child. And she's been awesome in everything ever since.
2. Chae Soo Bin: I've loved her in everything, even in some rather dreadful dramas (looking at you here, Where Stars Land), but I especially liked her in Piece of Your Mind, and Sweet and Sour, the new recent film with Jang Ki-Yong.
3. Son Ye Jin: Well, what can you say, she's a goddess. Not young any more, but still perfect in just about any role. Just a couple of years ago she played some scenes as a young college student in Be With You, with is a really beautiful film if you haven't seen it.
Also, an honorable mention to IU, although I've only seen her in one thing, her brilliant performance in My Mister. I tried to start some others with her, but had to stop, as I couldn't see her as anyone else. She will always be Lee Ji An for me.