Would say Ouran but I'm not sure I consider that shoujo.
This overly underrated anime right here.
Usagi Drop (josei but whatever)
Utena for anime.
For manga, maybe Basara or Fruits Basket.
Nike declares her song is super important and requires a complicated ritual prep in episode 1.
Then proceeds to sing it for every tiny buggerall reason every goddamn episode.
It was great since I've got tired form first loves, misunderstandings and blushings.
Characters are not one dimensional, has their stories and past and interesting development.
Pic related holds a soft place in my heart. Glass Mask is great too.
What shoujo manga would you want to see get anime adaptation?