most difficult faction?

[Rurik the Chieftain]

Try Spain, Numidia and Dacia.

[Count Belisarius]

I agree with most of the above comments. I would add Thrace to the mix. Very tough strategic position, strong neighbors, and poor starting troop mix. Last time I played Thrace, Macedonian Light Lancers just overwhelmed my falxmen. But overall, the Parthians get my vote for hardest faction to play. Nervos belli, pecuniam infinitam.

[MoROmeTe]

The Dacians aren't that hard, although I think that the game should only be played on Emperor (that's VH for vanilla RTW) for the Campaign Map and on Consul( that's H) for the battles. On this level Dacia is really interesting to play.

Hardest faction? I'd go for Numidia. Parthia can kick ass if you manage to take out the Seleucids rather quickly, beginning with Seleukia. The Numidians are much harder.