Order, without a doubt.
You've got Social Order's garrison bonus along with essentially 3 mobile Towers of Complacency (4 counting Sphener), meaning you'll almost never have to deal with unhappiness. On top of that, you get Basilicas and Law mana for running a large empire with little maintenance, you have guaranteed access to Paladins for direct combat, you're immune to hell terrain, and it's not as much of a tech investment as the other late-game religions (Empyrean, I'm looking at you).
Veil is powerful, too, but it has a lot of drawbacks (unhappiness, hell terrain, lack of gold, armageddon, etc.) that the Order deals with more easily.
Empyrean is great for military but otherwise is pretty bland. Their sole advantage in peacetime is the Overcouncil but that can be used under any religion, and even then the council's most meaningful action is -you guessed it- declaring war.