The last Airbender
For me there is only one true Avatar and it's most certainly not this sci-fi remake of Pocahontas (personal opinion).
And that involves the story around Aang, the last Airbender, from a fantasy world where there are only 4 nations which each have mastered control ("bending") over one of the four ancient elements: Fire, Water, Earth and Air.
What makes this animation series a true classic in my opinion is the way its being played out. It touches "adult" topics such as things like oppression, warfare, the ancient "power corrupts and total power corrupts totally", but also gives us an insight to the wide variety in a world. Different nations, different people, different habits.
The best parts about this series, in my opinion of course, are the fact that it actually has an ending, the eye for details (it spans 3 seasons, which matches the 3 nations (the Air nomads had been wiped out by the Fire nation)) and...
So you follow the series and learn almost from the first episode that the fire nation are the main oppressors here. They murdered the Air nomads for no other reason but to stop the next incarnation of the Avatar. Then they started invading the nations of the water tribes and eventually also directly assaulted the Earth nation in an attack which would most likely have taken millions of casualties (the "Scorched Earth" technique).
So what would you expect when you'd visit this fire nation? A whole country of power hungry and war-eager people ready to chop of the heads of any outlander?
More than often the activities of a government don't always have the support of all their citizens. Something which Avatar managed to bring to the viewers attention in a more than excellent way. There is a war? People go on with their lives, the only thing they do hope for is that their family will be home soon.
THAT is Avatar for me.