What is the best way to gauge 2nd gen success?

BigBang was on top #1.

Album sales for suju, snsd, and some other groups compared to their rivals.

Also chart, popularity, and touring.

Well, I guess it'd be a combination of sales (physical+digital), popularity in SK (Gallup, CF deals, song charting), international recognition (youtube views).

On that note, for the boy groups it'd be TVXQ #1 for early 2nd gen era and Big Bang for mid to late 2nd gen era.

For the girl groups, it'd be Wonder Girls #1 for early 2nd gen era and SNSD for mid to late 2nd gen era.

Well TVXQ ruled 2004-2008. While SJ/BB ruled 2009-2015.