If I have Zhonyas Hourglass and all units die at a battlefield due to combat, who decides what unit is 'revived'?

Ruling: When a replacement effect would replace multiple events at once (multiple units dying), the controller of the replacement effect decides which event it will replace.

Nuances:

• The unit is not technically "revived" because the replacement effect prevents the death from happening in the first place
• If multiple replacement effects would attempt to replace the same singular event (like Zhonyas and GA on one unit), you choose which replacement effect happens first
• When one replacement effect resolves (healing, exhausting, and recalling the unit), the second replacement effect cannot trigger because the unit is no longer "dying" - it's being healed/exhausted/recalled, which isn't a valid event to replace
• Only one of the replacement effects will go off (your choice), the other doesn't trigger