Sasanids by far...Great economy and cheap cavalry armies...
In my campaign,I played agressively, quickly destroyed armenian kingdom and Lakhmids (the southers arabic faction). But be carefull about White huns at north east. Build cheap spears and arches to defend cities and quickly build stone walls. Since white huns armies consits of mainly cavalry, they cannot assult towns. Later, after you buld up strong armies, you can defeat them easily.
As Sasanisd your most powerful enemy is Rome Orientalis. They field massive armies, stack after stack pours in. There are several ways to fight with them.
One way, Deadly onagers...they are pretty devastating. Most armies of Rome consists of mainy pretty tough infantry. But they lock shields too early and tire themselves and become obvious targets to your onagers..

My record is 659 kills with one onager
Another method it that I built armies mainly step horse archers which take 0 turn and 20 upkeep and some savaran cavalry. And do as much as damage to romans with those cheap armies and hit the weakened romans hard with your main army.
Also, try to hold birdges with onagers, just stand there and let them come. Bombard them while they are trying to pass bridge..
Good Luck...
