Mercs?

[Red Harvest]

With mercs I find it is not so much what is best, but what does your faction really need or lack? Balearics are the mainstay for my Carthaginian and Spanish missile corps and I use them carefully. Merc Numidian cav are very useful in the Africa (and a must for chasing down Numidians.) Cretan archers are good all around, and should be snapped up wherever they are found, if not needed by you then simply to deny the AI such a potent unit.

As Roman factions early on I snatch up Samnites and sometimes hoplites for anti cavalry--but I modified the Samnites and other non-phalanx spears from +4 to +8 mount effects. The vanilla Samnites should be better than they are vs. cav (same is true for most non-phalanx spears.) I exclude units like townwatch and the like from mount effect adjustment because they are peasant level.

As Germania I desperately need cav early on, so I buy all the barbarian cav I can find.

[KiOwA]

Rhodian and Belaric merc slingers can also wreak some real havoc on enemy forces, if you learn to position them in front of your units so they don't cut a swathe through YOUR units.