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.
Onto the less personally emotionally loaded series and mangaka you would like:
Riyoko Ikeda: Besides Oscar, I thought “Oniisama E…” was lovely, but in terms of drama it’s similar to “Heart of Thomas”. My secret favorite of hers is “Ten no Hate Made”, another historical drama, this time with Poland and with a fictionalized version of Jozéf Poniatowski’s and Tadeusz Kosciuszko’s life. It has a lot of politics in it and a little less story, but I liked it nevertheless.
Clamp: I am actually not a big fan of Clamp. They also don’t have all that many great shoujo, most of what they are good at is shounen. Well, “Card Captor Sakura” is cute and their art is wonderful, but for me, that’s about it. Okay, there is an exception: I love, love, love “Magic Knights Rayearth” because it’s the series that got me into manga in the first place. Plus it’s fairly short.
Fuyumi Souryo: Her non-shoujo (“Eternal Sabbath”, “Cesare”) are her best series while “Mars”, her longest shoujo series, is kind of stupid. But her short stories are brilliant too (“Taiyou no Ijiwaru”) and I used to be a big fan.
Mari Okazaki: She has a few masterpieces and a few silly stories. Perhaps the best thing to recommend is her short story collection, “Yawarakai Kara” or “Soft Shell”. I also liked “Shutter Love” a lot (but the French translation of my copy is, in my opinion, better than the scanlation). Another friend who read lots of shoujo manga also recommends “Suppli” which I liked as well.
George Asakura: Actually a woman. I am strangely fond of her style, but her stories are all kind of crazy. I liked her short story “Suimitsutou no Yoru” but that’s about it.
Kaoru Fujiwara: Well, I really like her art. Apart from that, her manga are not all that great in my opinion, though “Fetish”, a short story collection, has a few intriguing stories.
Moto Hagio: She is probably, without a doubt, the most shining shoujo mangaka next to Ikeda. “Hanshin” and “Slow Down” were good stories, and apart from that I haven’t actually read much yet. The friend of mine (mentioned in my previous comment) is an incredible fan of “Poe no Ichizoku”. On a side note: Back then, when I actually *was* indeed just a management undergrad, I wrote this post, which I think reflects my view on manga and its academic treatise pretty well. Almost every manga mentioned in the posting is an important classic, and now that I am actually an active part of (not-management) academia, my views on it are still pretty much the same.
With that said, onto some manga titles:
Glass Mask: I love stage plays, and even though “Glass Mask” is a cheesy melodrama sometimes, I thought it was incredibly powerful at many instances. It’s a very, very long series though, unfortunately.
Natsume Yuujinchou: The anime reflects pretty well what the manga is doing, yet the mangaka is actually quite important. The manga is also a little less cute and more artistic, in my opinion.
Hyakkikyou Shou: If you are not averse to fantasy, this series is a must. It’s long but the stories are unrelated to each other. It’s a little like Mushishi for girls (except nothing about the series is particularly girly).
Nodame Cantabile: A modern manga, and while I have stopped reading it after volume 10, it is perhaps the best modern manga out there. Light-hearted, with funny main characters, many lovely, fairly multi-faceted side characters and great dynamics between the main couple. Another manga by the same author is “Tensai Family Company” which is almost just as awesome.
Sailor Moon: You probably have read this already, but it does indeed belong onto this list.
Basara: Again, a long series. This one is a fantasy story and has a more intriguing and fascinating story than . It’s one of those series that puts their characters into extreme situations and thus shows an incredible depth. Sarasa is the only female lead character with true leader qualities, you can’t easily beat that.
A shorter, similar but slightly less good manga by the same mangaka is “7 Seeds”.
Kodomo no Omocha: I think Miho Obana is a genius, and this series is her masterpiece. It involves young children but their characterization is surprisingly well-executed and deep.
I like some of her short stories too, like “Mizu no Yakata” and “Pochi”.
Houkago Hokenshitsu: I stopped reading this manga myself, but its beginning was extremely promising. The same goes for “Diamond Head” which I used to love back in the day, but don’t actually remember the story anymore.
Kimi wa Pet: For a slightly older audience, the main character is a successful career woman who actually is shy and has extremely low self-esteem. In a nutshell, she reminds me of myself.
Switch: By Karin Mochizuki. I haven’t read it for years, and I think it’s a great, 2-volume pearl that nobody really knows about. It’s a student-teacher relationship which I usually dislike, but in this case, it’s very subtle and beautifully done. No other manga is incorporating the philosophy “Less is more” better than this one, and for that reason, it probably has the most beautiful kiss scene I have ever seen in a manga.
Unless my tastes have changed drastically in the last 5 years, I wholeheartedly recommend this.
Tokimeki Tonight: An old, pretty shoujo and a lot of fun so far.
Oh God, writing this totally made me want to read all those manga now. XD