And im not saying that all other barbarian factions suck. I have played the Getai, BOTH Gauls, the Lusotannan and the Casse. However the only faction that i keep coming back to again and again are teh Sweboz. They are just fun to play.
It a slow start faction, allows for plenty of build time, and has VERY powerful infantry. Once u get their economy going you can pretty much steamroll everyone around you. Also, like most barbarian factions, almost all of their units throw stuff at your opponents.
All in all its a very cool faction to play as.