This is across all generations. The criteria will not be defined, because there is no standard baseline that can be applied across every era of K-Pop.
If you have confidence in your girl group knowledge like me, you can put your list in order.
For context: This list should be put together as if a normie walked up to you and asked who the most iconic girl groups in the history of the industry are.
Now, I present my infallible list.
1. SNSD
2. BLACKPINK
3. Twice
4. NewJeans
5. S.E.S.
6. 2NE1
7. Wonder Girls
8. Red Velvet
9. MAMAMOO
10. Sistar/Kara
I left off some amazing girl groups, so this was actually very, very difficult
AOA, f(x), APINK, T-ara, GFriend, Kara and Miss A just to name a few.
A deserving girl group gets left off no matter what.
MY criteria ranges from number of hit songs, cumulative statistics, peak popularity, fandom size, and longevity, along with the more nebulous category of overall impact
The only generation I didn't experience personally was the first one.
God, not having Kara on the list hurts my soul. They were SO popular at their peaks, (WonSoKa) and the first Korean girl group to become massively popular in Japan. They were the first to hit many major milestones that fans take for granted today....Ah I can't do it. I'm going to set a bad precedent and cheat.
Anyway, I could write multiple essays about the groups I did choose and the order, so I'll stop here.
I want to see some lists