What started it all - the manga by Naoko Takeuchi. Beautifully put together, and one of the best known shoujo mangas in history. I can't really decide if I like the anime or manga more... The manga storyline isn't as detailed as the anime - less comedy and minor baddies to fight, but it's also a lot less repetitive. Anyways, definitely worth a read if you can get hold of it, which shouldn't be too hard. The summaries below are arranged by their english release.
Original Series
Sailor Moon SuperS/Stars
upcoming, if/when I get copies...