These groups actually focus on vocals, it's their main strenght or at least one of them and each member could receive worthy praise if you asked them to sing you a song out of the blue.
They debuted in 2003 under YG Entertainment (yes) and they are maybe the vocal standard for girl groups. They were active until 2012 but they had a revival around 2021. All the vocal groups listed in this thread impress me but Big Mama are something else.
They debuted in 2005 and they are nothing more than SM best vocal girl group. They never quite made it but they were not complete nugus. I always come back to their acapella moments in particular their Can't Help Falling In Love cover.
They debuted in 2006 and became a solid mid-tier group. They often produced their own songs and JeA is still a songwriter for others groups or OST. You know that you were up for a great start when JeA selected the members herself. Thye only had to train for three months and Gain released a ballad album before officially debuting with the group. They are kinda the most idol-like group of the list so far but a lot of their songs are empowering and challenge conventions.
They debuted in 2006 and were a sympathetic vocal trio. All the members left the group and were replaced in 2012 by a new generation called The SeeYa who were also talented.
Now let's talk about 2,5 or third generation groups with Spica who debuted in 2012. They are my second favorite vocal group. I really love their retro pop sound and the members all have a great voice. Two of them are still around and debuted as a duo called Keembo in 2020. Their It's raining men cover is legendary.
6. Kiss&Cry
They debuted in 2013 and were disbanded in 2014 because they had management issues but if you were already into Kpop back then, you migh remember their song Domino Game. Their Lady Marmalade cover was another great vocal moment. Their main singer Dia had a solo career before and after their disbandment.
7. Mamamoo
Last but not least, Mamamoo. The group that I don't need to present. They are really dear to my heart. If you want talent and a charismatic group who have a lot of fun on stage, then Mamamoo is made for you.