I think the last episode, part 4, is the best. I think it might be one of the best endings to a show I have ever seen and in basically every way, it was just fun to watch. Maximum Fire-bending? Fully-realized Aang? Katara/Zuko vs Azula? Oh god I am getting excited just thinking about it, but I am here to talk about part 2. If I had to pick one single episode that "defines" why AtLA is great, I wouldn't choose part 4 even though it was the most satisfying and beautiful, I would say part 2. The reason being is because it shows just how in-depth AtLA is, how truly "adult" it is, and brings to the forefront my favorite motif of the show, the "passing of the torch" from the 'Old Masters' (episode namesake) to the 'New Masters.' Life is cyclical, and there will always be "once in a generation" moments in well... every generation. You can say this moment now is the most important in history, but the next great moment may say the same thing. The 'Old Masters' are the embodiment of the spirit of Team Avatar. They have Sword, Fire, Water, Earth, and would have Air if they could. It is the same kind of group the 'Young Masters' would be to Korra and her team. It is really amazing getting to see the generational gap transcend the show's storyline. If that wasn't enough, this is Aang's greatest trial. He has lived 1,000 lives, and in 999 of them, he would "do what has to be done" with the Fire Lord, and his previous lives tell him that, but that isn't Aang. Aang is so pure of heart, he refuses and finds another way, despite it being impossible, despite everyone, including himself technically, that he should. Inn every TV show, every movie, and in all of our real lives we have to make decisions, tough decisions and we battle the voices of others and the voices of ourselves on what to do, but ultimately, you have to make the decision. I watched this show when it came out, and I was 11-14, and I am now 26 rewatching it on Netflix. It is frankly even better now. I guarantee if you watch this show, you will love it, and this episode will always be a top tier episode.
