My vote would be for the Grigori, followed by Kuriotates and Hippus.
Grigori ideas:
New unit: Chaplain. Requires priesthood, adventurer's good.
Str 4, starts with scourge,inquisitor. 150 hammers. Disciple unit
targets disciple units first
New civic: humanism, requires grigori, priesthood
Effects: no non-state religion spread
+2
for each state religion in a city
no priests allowed
+2
in all cities +10% beaker +10% GPP
No upkeep
I'd also throw in the random race idea from Tweakmod for the Grigori
Grigori ideas:
New unit: Chaplain. Requires priesthood, adventurer's good.
Str 4, starts with scourge,inquisitor. 150 hammers. Disciple unit
targets disciple units first
New civic: humanism, requires grigori, priesthood
Effects: no non-state religion spread
+2

no priests allowed
+2

No upkeep
I'd also throw in the random race idea from Tweakmod for the Grigori