To say the all-woman Japanese creative group CLAMP are giants in the comics world is to make a gross understatement. Producing professional work since the late 1980s, their 20+ series have sold nearly 100 million copies and many have received animated adaptations. (Compare this to the manga biz as a whole, where most artists are lucky to get just one hit series and adaptations are once-in-a-lifetime events for all but the biggest creators.)
Their first real megahit - and, less than coincidentally, a series that's recently gotten a fancy rerelease here in the States - is Magic Knight Rayearth. It starts off with a typical teen fantasy plot: three girls are transported into a magical realm, where they have been summoned to save a princess. From there, though, it takes off in its own direction, with the twists and turns CLAMP has become famous for.
It's a great introduction to their work - and fantasy manga in general - with beautifully detailed artwork and an epic story.