I personally prefer hellenic factions, as they are the typical "civilized" lot during that time. They have a very varied roster, from old school hoplite troops to Mega Alexandros' phalangitai, then the rise of more mobile thueros troops (no) thanks to the keltoi invasions. Their recruitment range is huge, so there's generally no shortage of hellenes to replentish one's core ranks with. They also have a rather varied array of regional that can be easily intergrated into one's armies. Finally, they have the most well known history and culture we can relate to, which gives them endless possibilities of role-play scenarios.

Second after that would be the various 'Barbarian' factions, they each have their unique culture and playing style. Far from the sterotypical unwashed orkish fiends (Waaagh!!! ), they are arguably less barbaric than the so-called civilized (humies!) city-dwellers. Their troops are generally highly specialized, each fulfilling a role of sorts in the battlefield. One of the things I enjoy most as the Keltoi is to sack and pillage Roma, giving it back to them, gift the territory between them to an ally, then coming back to sack them again 10 years later. A historical irony of sort.

I do not like Romans, they are too generic and boring. If I want to play the romans, I would've gotten RSII or RTR. Also, they're culture is a one-off in the entire game, making expansion a somewhat annoying task as time runs on. They are neither greek nor 'barbarian', and in some aspects, a lot more barbaric than the keltoi, Iberioi and getai to name a few. The only thing worse than them gameplay-wise are the steppe factions, lots of HA fun in the begining, then grinding to repetitiveness from the middle onwards. I blame the limits of the engines.