Groups whose music has only gotten better since debut?

Twice

Quote from kirishiki

DreamNote

MY HERO! THANK YOU!

Also, yes. Their earlier stuff is great, but Wish and Ghost is a hard combo to beat.

Lovelyz

wouldn't everybody just list their own favs?

I think most groups sound better as they go along, as the artists become better performers with better knowledge of their own capabilities and more fully formed skills. At debut, most are trying to match the company’s view or expectation, which may not always be the style, tone or even pitch that works best for them. Or the idol may strengthen their skills to be better and tackle a wider range or more difficult raps or melodies . Your world view has expanded at 26 from where it was when you are 19 and you can add more thoughtfulness, weariness, poignancy and emotion to your delivery of a song.

Twice ♡

well the question is flawed, because the only way for their music to never get worse is to never release a title track that is less interesting than one before it, which gets exponentially harder the more songs they release

I can't really think of any groups that never went down