That's a pretty interesting question. For me it has to be the 1990's (Sailor FRIKKIN' Moon's in here!), I adore the stylish animation of that decade in the maho shojo genre we can also see that in Utena. Well, I love 90's anime aesthetic in general, so... yeah, easy choice.
Between the end of the 90's and beggining of the 2000's I think we had amazing magical girl stories. Cardcaptor Sakura comes to mind...
But I also find the 2000's extremely important, since it's the last decade with cute innocent magical girls that were big hits, and before the "dark edgy deconstruction" age came to be.
We had shows that didn't shy away from some darker elements (Princess Tutu) and showed us more "realistic" settings and a lot of action (Like Pretty Cure) without turning into soulless gore-fests.