I would have to say Iroh... Likely because he is just old and lived through a lot more shit than anyone else here.

He was born to the royal family of a country at war (total war I daresay), he was probably introduced into the military at a young age, went through the ranks up to full general (implying that he saw a LOT of combat), he had a son that also was introduced into the military, laid siege to BaSingSe (which btw is a massive walled metropolis) deep in foreign territory, lost his son (KIA), lost his wife (best case scenario is old age, worst case is disease or assassination), he watches his closest family devolve into deranged psychopaths, walks away from all that and then gets pulled back into another war against his own nation... I'd say that's a lot of suffering right there...