Considering all aspects (physical sales, national and international popularity/success, digital success, charting, etc.)
I'm going to assume SNSD is an easy first, but how do the others stack up?
I know KARA had monstrous Japanese album sales, T-ARA was popular in China, and SISTAR/2NE1/Wonder Girls were consistently strong on digitals, what about miss A, f(x), Girls Day and APINK? And how would all of them rank in terms of overall success for TOP 10 Girl group in 2nd generation?