Each period had their best girls. Since I haven't listened to KPOP for a while, Ill leave the last decade to the rest and focus on some old school ones.

First Gen:
Baby Vox's Lee Hee Jin & Yoon Eun Hye

(Best Music: As One and BoA)

Second Gen:
T-ara - Hahm Eun Jung
4 Minute - Hyuna
Girls Generation - Kim Taeyeon
KARA - Nicole

(Best Music: SeeYa)