Who is the stronger Avatar - Aang of Avatar: The Last Airbender or Korra of The Legend of Korra? On both shows, "bending"consists of the ability to telekinetically master the element of one's nation of birth, either Water, Earth, Fire, or Air, with each being taught as a discipline of martial arts. The Avatar is unique by being the only individual with the capability to command every element, which both Aang and Korra learn to do on their respective shows.
There's an argument to be made that Korra was the stronger Avatar, due to her being more naturally physical than Aang. Whereas Avatar followed Aang's training to become skilled every element but his native one of air, Korra was well ahead of Aang in this side of Avatar training. In her very first scene on The Legend of Korra, the young Avatar was in complete control of three out of four of the elements while she was still a small child, with air being the only element Korra could not yet bend.
By the same token, Airbending proved to be a real challenge for Korra when she was taken under the apprenticeship of Aang's son, Tenzin. Though she would come to master it, her training with Tenzin also showed that Korra was much more prone to frustration and letting her temper get the best of her than Aang was. At the same time, Aang grew up as a Southern Air Temple monk, and while not incapable of losing his own temper, he had an ability to keep his emotions in check that Korra lacked. However, the deciding factor lies in Aang and Korra's proficiency with the spiritual component of being the Avatar, and here, Aang was unquestionably the better of the two.
Though becoming skilled with every element is an essential step of the Avatar being fully realized, the spiritual core of being the Avatar is where one's power is truly based. With the Avatar channeling the Spirit of Light, Raava, along with his or her past lives, Aang possessed a greater understanding of this being the foundation of his Avatarhood than Korra. This also goes back to Aang's upbringing in the Southern Air Temple, which gave him an ideal basis to learn the ropes of being the Avatar.
At the same time, Korra was more likely than Aang to fall victim to being blinded by anger in difficult circumstances, and this would prove to be a challenge for her that went hand-in-hand with mastering Airbending and the Avatar's spiritual essence. Still, how much more adept Aang was in the Avatar's spiritual center can be seen in his finale battle with Fire Lord Ozai. Despite being in the Avatar State, driven by his countless past lives and fully preparing to slay Ozai, Aang nevertheless had the self-control to stop himself from doing so.
Clearly, both Aang and Korra were very skilled as both fighters and Avatars, with Korra having far more natural talent as a warrior than Aang. However, while Aang took his time throughout Avatar: The Last Airbender to develop proficiency with each element, he also had a better understanding of the spiritual nature of the Avatar as being its the foundation than Korra in The Legend of Korra. Ultimately, this made Aang the more powerful of the two Avatars.