This is a no-brainer. Avatar Yanchen, of course! Aang is powerful (in the Avatar state) but she is stronger. If both of them clashed (which would never happen) she would have won hands down. Aren't Yanchen and Aang essentially the same person, by virtue of them both being Avatars? There's only one airbender, Aang wins by default. And besides he mastered airbending at a younger level than any other airbender ever. No actually Aang would just use Energybending and take away Avatar Yangchen's power. Yangchen definitely has a more unbendable spirit than Aang, overall, she looks tougher than Aang.