20Strong Enough: Katara
Katara may have been a kid when she first joined Aang's mission to master the four elements, but she quickly developed into a master waterbender. Katara was able to hold her own against fearsome antagonists like Azula and Zuko, sometimes even when Aang could not. She not only became a
master of the advanced waterbending combat, but also developed her skills in the rare waterbending subsets of healing and bloodbending.
She thinks on her feet in a fight, quickly strategizing how to use water, snow, and ice to her advantage. She picked up bloodbending with little training, and Aang and Korra have been subdued by bloodbending before, if only briefly. The woman who trained Aang in waterbending and took down Azula once and for all could certainly give the Avatar quite a fight.