Aang and Korra are both powerful Avatars, but Avatar: The Last Airbender world began in the mid-2000s and it is just as popular now as it was then. The original animated series become a defining show for an entire generation and an even wider demographic discovered Avatar once the show became available on Netflix in early 2020. But the franchise continued after The Last Airbender ended in 2008 with the launch of The Legend of Korra in 2012.
After viewers spent three seasons watching Aang master all four elements and bring peace to the world by defeating the Fire Nation, The Legend of Korra continued the story 70 years later. The series followed Korra, who became the next Avatar after Aang died, as she went on a very different journey as the Avatar. Thanks to both shows, Aang and Korra's abilities as the Avatar were thoroughly explored. While fans might be able to debate whether Aang or Korra is the more powerful Avatar, there is no debate who is the better bender.
Aang and Korra both mastered the main four forms of elemental bending, as well as energybending. Korra was able to do some bending with fire, water, and earth by the time she was 4 (showing her natural gift with bending), but Aang managed to learn water, earth, and firebending in one summer. It might have taken Korra longer to master the four elements, but her skill went deeper than Aang's ever did. Korra learned sub-bending skills like metalbending, healing, and spiritbending, while there are no signs that Aang mastered these abilities.
Korra's ability to learn more bending skills might be the state of the world that she grew up in compared to Aang. The Avatar world was very different and more advanced by The Legend of Korra, while Aang's time as the Avatar largely took place without any great mentors by his side. On the other hand, Korra benefited from the knowledge of Aang's time and friends, with Katara teaching her healing and Toph ing on metalbending - which was much more prevalent at that time. Meanwhile, Korra learned about spiritbending from her uncle, which wasn't created during Aang's lifetime.
There is a potential caveat to this discussion though, as there are decades of Aang's life when he would be in his prime as the Avatar that hasn't been explored. Aang made a few appearances in The Legend of Korra, but fleshing out his story wasn't the focus. Aang could've also learned metalbending and other sub-bending skills, but it's impossible to know for sure. Since the Avatar: The Last Airbender canon hasn't indicated that Aang mastered these abilities too, then Korra is indisputably the better bender.
