Best/Good Domain Choice for an Emissary Familiar?

[ATHATH]

Spoiler: Domain Influence (Sp or Su)

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At 3rd level, the emissary gains a spark of divine power from the patron that sent it. Choose one appropriate domain that grants a 1st-level domain power usable a number of times per day equal to 3 + the user’s Wisdom modifier. The emissary can use that power once per day.

This ability replaces deliver touch spells.

According to this (not sure how official it is), the familiar's effective level in Cleric for the purposes of the domain power is equal to your level.

So, what are some good choices for this ability?

[Florian]

Depends on how active you use the familiar during an encounter.
Luck is a very good support option, Rune is a very good tactical option, elemental domains are a nice little offensive option.