No one has suffered more in the ATLA/TLOK Universe than the Northern Air Temple imo.
☗An entire culture massacred by a bunch of Comet-enhanced Firebenders.
☗100 years later: Refugees polluting it, demolishing walls, statues, etc. Gas leak explosion destroys the lower part of the mountain.
☗70+ years later: Attacked once again. The Red Lotus capturing and holding the New Airbenders hostage. Combustion Bender playing sniper, blowing up portions of the temple, and shortly after crumbling at the hands of a suicidal Lavabender.