Best starting faction as something to move into barbarian factions would be Pontus. They have a pretty easy go of it early game where they are and tons of potential to form a large empire before facing off with some of the harder factions.

Odrysian or Tylis have some amazing shock infantry. Thracian warriors just tear stuff to shreds. Only downside is you start pretty close to the Greeks and relatively close to romans and the horse tribes.