Definitely the Getai.

Stopping the Hellenistic powers (Macedon, Seleukia, Ptolemies) is a lot harder than stopping Rome. In EB, Rome (when controlled by the AI) is a pathetic weakling, easily conquered even by the other AI factions - Lusotanns and Carthaginians conquer Rome all the time. But the Getai have a much tougher time against the Hellenistic phalanxes.