This decision is actually kind of difficult. If you think about it, Nicole and Jiyoung seemed to have roles that were really similar to their actual personalities. Throughout the whole series, Jiyoung played parts that emphasized her cuteness, which she was pretty good at. Nicole's role in the series was pretty similar, she did pretty funny things quite randomly.
My point is, it was probably harder for Gyuri/Seungyeon/Hara to act out of their character, but I think they did a good job nonetheless, especially for Gyuri. I thought this was because she sort of had to restrict herself in essentially acting as the "mother" (maybe she does that, idk), but it seems pretty difficult to do, considering her and Seungyeon are nearly of the same age.
Now having watched all the episodes, I think I'd have to say Nicole.