Sure, the series finale has a slight advantage in this ranking since it's the length of four separate episodes, but the primary reason Sozin's Comet gets the top spot on this list is because of how perfectly it resolves all the plotlines and character arcs that built up over three seasons.

The way in which the show effortlessly juggles this intense character study that deepens one of its 'villains' with a relatively realistic depiction of the global conflict's consequences is prime of example of how Avatar was more than just a kids' show. He started out the show as an antagonist and here he's forced to confront the consequences of his actions and how people perceive him as he's trying to figure out how he feels about himself. Up until this point, Avatar had done a good job of exploring the horrors of war, but this instance was particularly affecting because the Fire Nation weren't the villains of the story, indicating good and evil aren't as clear cut, and more importantly, we were seeing it through Zuko's eyes. After separating from his uncle, the wayward firebender finds himself in a poor Earth Kingdom village and decides to defend the innocent townspeople from the exploitative Earth Kingdom soldiers. Avatar goes full Western with this standalone tale about Zuko.

Watch avatar the last airbender book 3 episode 13