well, i think Greeks (koinon ellenoi) is an almost finished facion, they have their personalized CM interface with a whole set of new icons, and the units roaster seems almost complete to me, there isn't still a general custom skin, but all buildings and thinks are all very accurate, it also seems they have all the units cards etc. So try this faction, it is really enjoyabile and the Torakitai oplites are really nice.

according to what i read, it seems that in the next version there will be also spartan oplites (even if i really dont care of them as i think they are not necessary). Regarding traditionnal oplites there also the Chorintian. This is the most complete faction i tryied so far.

I also read that the seleucids (arkè seleukeia) is one of the most completed, but i haven't tryed them (since they are my worst enemies in my actual campaign)

Chartage has some cool units but the general bodyguards are still vanilla and the numidian cavalry is the worst skinned of the whole mod (but they are just a placeholder)

gallic tribes seems also near to be complete to me, but no faction seems to reach the koinon hellenoi accuracy and cleaness in my opinion so far

that what i can say from my actual experience