I think Fushigi Yuugi has got to be one of the shoujo series out there.
There is also everything else by Watase Yuu, Alice 19th, Ayashi no Ceres, and Imadoki; just to name a few.
Angel Sanctuary is also a REALLY great shoujo series.
Eventhough I don't really watch shoujo that much anymore, it's all shonen-ai and shonen stuff.
Fushigi Yuugi and Angel Sanctuary will always be at the top of my list. FY has this really great universal appeal, everyone has at least one character that they can't get enough of.
And with AS, there are so many things in it that are just so taboo in society that you can't help but love it.