Throughout Avatar: The Last Airbender, one of Aang's biggest goals was to master the four elements: air, water, earth, and fire. He had already learned how to airbend before the series began, and slowly learned the other three elements throughout his journey.
Aang learned how to waterbend from Pakku and Katara, with Toph becoming his earthbending teacher shortly after. Towards the end of the series, Zuko became Aang's firebending master, and was arguably the best teacher that the Avatar had.
6 By Being His Enemy First, Zuko Was Able To Help Aang Become A Better Fighter
Before Zuko became Aang's firebending master, they fought many times. While they didn't intend for it to be, all of their battles could be considered training, as without them, Aang wouldn't have been as good of a fighter. There were also times before they became friends when Zuko helped Aang, such as when he rescued him from Zhao or when he allowed Appa to escape Lake Laogai. If he hadn't, Aang would have been in a lot more danger and may not have been able to learn firebending from Zuko at all. However, Aang learned a lot from Zuko prior to them being on the same side, and it's these teachings that would eventually help Aang become a great Avatar.
5 He Left Everything Behind To Train Aang
For years, Zuko wanted nothing more than to have his honor so he could return to the Fire Nation and be with his family. However, once this dream came true, he realized that he had changed a lot over the years and that everything he worked so hard for no longer mattered to him. Now seeing the truth, Zuko understood how terrible of a father Ozai was to him and betrayed him in order to join Team Avatar. As a result, he left everything behind, including his home and his girlfriend, Mai. Through the sacrifices he made, Zuko proved how important it was for him to teach Aang.
4 He Brought Aang To The Sun Warriors
When Zuko first became Aang's firebending master, he lost the motivation he had that made him so powerful. Knowing that he wouldn't be able to teach Aang if this continued, the two of them went to the Sun Warriors' ancient city, hoping to learn more about the original source of firebending.
There, Zuko and Aang learned how to use firebending in a better way than they ever had before, with the Fire Nation's prince having a new motivation to give him strength. Zuko proved that there are times that the most helpful way to teach someone something is by learning it with them from a different perspective.
3 Zuko Had Aang Practice All The Time
One of the most important things when it comes to mastering something is practicing it. Leading up to Sozin's Comet, Zuko had Aang practice firebending as often as he could. Even when he had gone to The Boiling Rock with Sokka, Zuko instructed Aang to practice the moves he learned constantly. He also didn't like letting Aang take unnecessary breaks and worked with him a lot so he could become the best firebender he could be before fighting Ozai.
2 Zuko Showed Aang How To Redirect Lightning
One of Zuko's most powerful abilities is one that he learned from Iroh, how to redirect lightning. Iroh came up with the move by studying waterbenders, and by teaching it to Zuko, Iroh helped save his life. Before Zuko joined Team Avatar, he confronted his father, who shot lightning at him.
Zuko made sure to teach Iroh's technique to Aang in case Ozai tried to kill him the same way. Aang quickly realized how similar it was to waterbending, making it easier for him to learn. Seeing as how Aang nearly died when Azula struck him with lightning, it was very good for Aang to learn this.
1 He Challenged Aang's Beliefs
Rather than just teaching Aang how to firebend, he helped him become a greater Avatar and person by challenging his beliefs. While Aang felt like it was wrong to kill Ozai due to everything that he learned from the monks who raised him, Zuko thought that it was necessary. This motivated Zuko to be an even better teacher to Aang as he was concerned that he didn't have what it would take to defeat the Fire Lord. Aang's conflict also resulted in him meeting a lion turtle that helped him come up with a solution to his problem. Aang was able to beat Ozai by stripping him of his bending rather than killing him, which also taught Zuko that there were other ways to defeat an opponent. Together, they learned a lot from each other.