Depends who you are playing and what your objectives are. As a confirmed Roman campaigner and trying to follow a historicalish timeline then it's Iberia, the Luso are tough troops and the terrain is tough especially given your forces when you face them.
By the time you go east you will often have had Marian reforms then most factions are fairly easy, and later in the game the AI fields pretty poor stacks, only the Saba seem to mix things up.
Carthage, is annoying in taking those damn North African provinces that I don't want.
As for horse archers by the time my Marians face them the Testudo is a very useful arrow magnet until they run out of ammo.

However, now it's the Arche Seleukia's turn and things look like being a whole lot different, especially as I don't really like fighting with phalanxes