These are just the ones that come to mind right now:
After School After School’s astonishing dance skills became such a hot topic after their debut stage it was said they were the only group to beat SNSD in dance.
2NE1 The first *major debut of a girl crush group, 2NE1 subverted viewers expectations of a rookie girl group, as they emerged straight from the gate as badass unnies
Gfriend one of the few times a rookie from a small company debuted with a lot of buzz--and for good reason. Many were surprised that this fresh group, seemingly on little budget, come out with senior-level dance and vocal skills, and a catchy song.
Blackpink the debut that had people exclaiming “ah yg has done it again”. with similar fierceness as 2ne1 but with more a more subtle, girlish charm, Blackpink surprisingly delivered on the hype YG had been tediously building for the past 4 loooong years.
Finally, this last one is iconic for a different reason...
A.C.E. this boy group's debut had many viewers scratching their heads and checking their eyesight as the boys bravely dawned short-shorts that were deemed "underwear" by many viewers
