SalKing2 has got completely it!
1. Aang not being able to have a normal, easy childhood, starting at the young, in-experienced age of 12.
2. Aang finding out that he is the last of his kind, and that everyone he had ever loved died in an ambush, done by people looking specifically for him.
3. Waking up in a ball of ice, and finding out he's been frozen for 100 years.
4 And all the struggles we have seen him go threw in the series.
Kyoshi is a close second though.
1. She lived for over 200 years, spending the majority of that time as the Avatar, protecting the world, and then was thought of as a killer when she died...
Kuruk and Roku are tied.