Ghazan committed suicide because he realized Ming-Hua was dead, and was upset about it

Definitely Noatak and Tarrlok, their father raised them to take revenge, and their “mission” was so ingrained into the both of them that they unwittingly carried it out anyway, in spite of them disowning him and their own motivations. What’s especially sad was that Tarrlok new that his brother would never stop no matter what, and the only why their father’s grudge would end, was with their deaths. These brothers never had a chance.

• Noatak,Tarrlok & P'li were blown up

• UnaVaata was erased from existence

• Aiwei was severed from his body and trapped in his madness for eternity in the spirit world

• Ming-Hua was electrocuted to death

• Ghazan was crushed/burned by lava

So I would put my vote on Ghazan

It was either between Noatak and Torlock or Ghzan because they both committed suicide for depression reasons which I can relate to. And that makes it even sadder. I cannot be the only who got caught off guard by Noatak and Torlock’s Death