Pahlava & Parthian REFORM !

[athanaric]

In short you get a New Barrack, in which you can train quite a lot of troops one of which is the strongest regular unit in game, the Grivpanvar. You also get some nice buildings that improve health , Law, happiness and give you an experience bonus for your troops.

You get to have the third strongest economy and infrastructure in the game (after Rome and Carthage, but really almost on the same level) instead of the pitiful one the regular pastoralism would allow you to have. Meaning you can actually build an empire with Pahlava, unlike Saka and Sauromatae, who are terrible in late game. And yeah, barracks.
Here's some of the units you can only recruit with reformed government:

Spoiler Alert, click show to read: