Does activating Zhonya's force a player to save a unit that gets assigned damage first with Tank, or can they choose which unit to save when multiple units die simultaneously?

Ruling: Although Tank requires assigning lethal damage to that unit first, all damage is dealt simultaneously, so all units die at the same time. Zhonya's controller can choose which unit to save.

Nuances:

• Zhonya's is a replacement effect, not a trigger
• You must flip Zhonya's before the unit dies
• Location is irrelevant - if a unit dies at any location, you can use Zhonya's to save a unit anywhere because the gear comes out from hidden and is played to the base
• If Zhonya's is hidden and a unit dies, it does not automatically trigger the hidden Zhonya's