Honestly, Jiyeon does not look THAT great without make up, even though her fans will say she does. Because her eyes are very close together, she needs the eyeliner and eyeshadow to make them look further apart. When she has no makeup you can really notice how close together they are.
Eunjung is stunning without makeup though. I think Eunjung is one of the top "no makeup" idols. Eunjung's features are ALL natural -- big eyes, big lips. She doesn't need makeup to make her eyes or lips or anything seem bigger or prettier, because they are naturally that way, and her hair is always very simple to, so when she removes her makeup she looks basically the same.
Eunjung = stunning!