When an opponent controls Zhonia and two creatures that both die simultaneously from combat damage, who decides which creature Zhonia's effect applies to?

Ruling: The opponent (defender) decides which creature Zhonia's effect applies to, because they are the owner of the replacement effects happening simultaneously.

Sequence:

• Attacker assigns combat damage order (like "tank" rules when there isn't enough might to kill all units)
• All creatures that receive lethal damage die simultaneously in cleanup
• The opponent who owns Zhonia chooses which of their dying creatures the replacement effect applies to

Nuances:

• It doesn't matter who the active player or attacker is; the owner of the replacement effect makes the choice
• Damage assignment order (tank rules) only matters when there isn't enough might to kill all units, but doesn't determine the order of death