Himitsu no Akko-chan. Basically the first magical girl. Well, anime-wise Mahoutsukai Sally predates it, but the Akko-chan manga is older than Sally’s (or Sunny’s, rather). It also introduced the idea that the basic power of magical girls is transforming (mind you, not yet into a combat form), which is the standard for magical girls to this day. Even Sailor Moon, which made a more superhero-like transformation the norm (though Cutie Honey did it earlier), had the Disguise Pen that worked similarly to Akko-chan’s mirror. The current Precure series, Star Detective Precure, has something similar too. And the also currently-running LuluttoLilly features a more free-form kind of magic, but even there it’s mostly used to transform into older girls.