This question is really subjective and depends highly on what you are trying to accomplish. It also depends on whether you are playing multiplayer or not. My commentary here applies to multiplayer only:

-The AS takes the cake for most diverse army options and general flexibility, with Carthage, Baktria, and Rome as the runners-up in some order.
-The "barbarian" factions (with the exception of the Lustanians) have the most offensive power, with the Sweboz being the greatest among these in aggressiveness of playstyle, and the Getai being the most flexible
-Roman units have the most staying power in general, and are probably the most predictable and reliable troops available