Originally this was supposed to be just another comment to this question, but the list quickly grew out of hand. So, I decided to make a post out of it.
In this genre, it so happens that longer series are most often better than the short ones. After all, if a series is popular it typically gets long. People are fickle, so if a series is long, there are good chances that it really is high quality. For me, recommending shoujo is a big fight between series that I thought are objectively good (such as Moto Hagio’s manga) or those who left me in tears at some point in my life (such as “Parfait Tic” which, for many reasons, will not be included in this list).
I also don’t know that much about the Year 24 group except for their most famous ones, but this list can serve as a good reference for what is out there.
Kare Kano: My favorite shoujo of all times. It’s relatively modern but not fancy-looking or anything. It also doesn’t have the best art in the world especially in the first volumes and its premise does not strike you as special. Nevertheless, it was widely popular, especially with the anime and is generally a very pure romance.
“Paradise Kiss” and “Gokinjo Monogatari” are significantly shorter, finished and both have nice stories as well.
Nana: Before Kare Kano, my favorite shoujo. After volume 8 the story becomes fairly erratic and shallow, but back then, when the series was not as popular, it had many elements almost every girl could associate herself with, namely the obsession with a man (or men in general). The series hits the sad truth that as long as a woman is heterosexual, no matter what else she does or what her character is, her world revolves around men in some form or another.
Honey & Clover: H&C is Nana for older girls, more or less. It’s a little more light-hearted and, if you ask me, much more oblivious and naive about relationships, but it hits home pretty well too with its bittersweet aftertaste. Another thing it does well is the depiction of love that can happen in a moment and last throughout the whole series. People move on, but very slowly, and almost no other series does it as well as this one.