For girls, SNSD, no need to explain.
For boys, I'm torn between TVXQ and H.O.T and I can see most people here if not all weren't into kpop at their peak if they said other answers. When TVXQ was at their peak, easily 8/10 person was cassiopeia (tvxq fan) and they opened the japanese market. Only seeing how even 20 years later they can fill stadium in Japan even individually said all. For H.O.T, they were a fever. Everyone and their mom knew them. They could sell easily 1M album without multiple copies or people bulk buying them as now only with their korean fans. Crazy and ironical how their 2 best boy group were wasted by SM and ended to disband.
