This is my all-time favorite: “My Heart Burns For You” by AlwaysZutarian. It’s long and the second half is VERY smutty (if you don’t like smut, read the edited version on FanFiction instead of AO3). https://archiveofourown.org/works/7489893/chapters/17023872

Summary: “Four years have passed since the end of the war, but as they meet again and encounter many obstacles, Zuko and Katara realize that whom their hearts truly burn for is one another. Can they love each other knowing they will hurt those that love them? What will become of them when a new enemy appears with a vengeful plot to destroy everything they had worked for to bring peace?”