There have been so many, but I gotta pick these two since they actually had a huge impact on me while growing up:
Alanna from the Song of the Lioness Quartet by Tamora Pierce:
Because growing up, she set the standard for strong female warrior character.
and
Nausicaa (as @eslameldeeb and @TheYuriCanon have mentioned) by Miyazaki:
Because she displayed a different kind of strength and leadership.
Again, there have been so many badass female characters since (yes, Game of Thrones, I'm looking at you), but these two and their stories really stuck with me throughout my life and made an impact on how I viewed strength. 