If enemy has Guardian Angel and Zhonya, which one gets used first?

When a unit has both Zhonya's Hourglass on the board and Guardian Angel equipped, and that unit would die, you (as the controlling player) choose which replacement effect to apply.

According to the established ruling for this interaction:

• You choose which effect to apply: Because both are replacement effects that trigger on the same event (the unit would die), you decide which one resolves first.
• If you choose Guardian Angel: Guardian Angel is killed instead, and the unit is healed, exhausted, and recalled to base. Zhonya's Hourglass does not trigger because the unit never died.
• If you choose Zhonya's Hourglass: The unit is recalled exhausted, and Guardian Angel does not trigger because the unit never died. Guardian Angel remains equipped to the unit at the base.

Important Nuances:

• Replacement Effects: Once one replacement effect resolves and prevents the death, the other effect no longer has a valid trigger condition.
• Guardian Angel's Mechanic: Guardian Angel's effect kills the gear itself, redirecting the death to the equipment rather than preventing it entirely.

This is based on the standard rules for replacement effects where the controller of the effects chooses the order of application when multiple effects would replace the same event.