Loona has reached well beyond the confines of typical kpop sounds since their first solo release, incorporating influences from many genres outside of kpop and outside of Korean music altogether, and has possibly the most musically varied discography among all idol groups without having sacrificed an ounce of production quality. Jaden Jeong, creative director at BBC, and production team MonoTree, among others, have done a great job pushing the boundaries of what kpop girl groups are expected to sound like.
Mamamoo has a long history of doing kpop on their own terms, presenting the tried and true girl crush concept in an altogether different skin, and after years of pop lightly touched by elements of jazz, r&b and dance music, they are now moving towards a much more mature sound reminiscent of soloists from other Korean musical genres, a growth well suited for the strength and versatility of their vocals and growing talents as songwriters. Of course, a great deal of the credit still goes to RBW's in-house production team as well as frequent songwriting collaborators like Cosmic Girl (aka Jooyi, formerly of RaNia).
AOA and 9Muses both have consistently great music across the majority of their respective discographies. For girl groups receiving the bulk of their attention for sexy concepts, it would have been easy for them to stuff their releases with meaningless filler, but each of their songs comes across as having been given the care and attention to detail that other groups might reserve strictly for their title tracks, and this consistency in quality is thanks in large part to longstanding partnerships with producers like Brave Bros (AOA) and Sweetune (9Muses).
EXID also boasts a very solid discography, borrowing liberally from a particular era of American hip hop and r&b that is near and dear to the production duo of Shinsadong Tiger and LE, and giving it a home in kpop without becoming purely retro or corrupting the source material beyond recognition. Their work, while sometimes predictable, is almost always tasteful, and it doesn't hurt to have some of the strongest vocals and rap in the game bringing life to an urban street style far more common to boy broups.