That would have to be Sailor Moon, the classic series. I grew up with it, and it inspired me in so many different ways. I have a sea of Otaku Senshi, several fanfics, and so much fanart. <3

I also really like Mahou Shoujo Madoka Magica (Puella Magi Madoka Magica). It has a darker theme, sort of like Sailor Moon versus Neon Genesis Evangelion.

In my younger days, I was a grand fan of Tokyo Mew Mew and Mermaid Melody as well. :)