My favorite unit would have to be the Tindanotae.



They are Asia's Gaesatae, and while I have never used Gaesatae, I have a feeling that Tindanotae are better.

They can make an enemy army rout as soon as they reach your line, and can frontally charge Argyraspides and obliterate them.

They aren't very common as mercenaries, and are as expensive as mercenary Tarentine cavalry, but a couple of units will be enough to take you to the Indos.