My favorite will always be the one that introduced me to manga when I was little: Sailor Moon
It was a bit more mature than than the original anime (Japanese, not the English dub), and I just loved the artwork. It's just special to me, because it was the first time a comic actually held my interest with its story and characters.
To me, the whole series is just gorgeous, and not just visually speaking.
I was SO happy to find out that they rereleased the series not too long ago. I was shocked to see it in the bookstore, as it had been out of print for years, and finally Stars is available for everyone! I still don't regret spending $40+ on old copies, though, but I would love to buy the new release with better translations and some colored artwork.
Thanks to this series, I discovered other magical girl series like Tokyo Mew Mew, Cardcaptor Sakura, and Magic Knight Rayearth, which are all great, too.