The first (maybe) kiss - The Cave of Two Lovers (2x02)
The entire episode has so much Kataang but the most significant moment comes when the two are trapped together in the cave and Katara suggests… an interesting solution to their problem. Which leads to some cute and hilarious pre-teen awkwardness between the two. This is really the first time we see Katara act in a romantic way towards Aang since her epiphany moment in The Fortuneteller and her blushing in this scene and in the end clearly shows that she doesn’t think of Aang as just a little brother. The actual “kiss” is not even shown but it is still a beautiful scene and a defining moment for the development of Kataang. It also introduced the lovely musical score (also known as the Kataang theme) that often played during important Kataang scenes, including their finale kiss.