I almost always play these factions...
Getai, Saruomatae, Saka Rauka, Lusotanna, Sab'yn, or Safot Softim biQarthadast

Getai are pretty well-rounded but they don't have any super-elite units that'll let you crush everybody, and their better units you'll need to pay accordingly for... and while it seems a little annoying that the general is a cavalry-archer, thats just part of the greatness of the Getai army...
while Arvenai, Aedui, Casse, and Sweboz lack a lot of great cavalry... Getai has skirmisher cavalry, charging cavalry, and horse-archers
those same barbarian factions have good shock infantry... but the Getai have the Drapnai (sp?) which if properly used will break most folks
Hellenics have hoplites and phalanxes...but the Getai have light and heavy phalangites as well
Romans have flexible troops...but the Thraikioi Peltasti can go toe-to-toe (until after Marian, then you better be ready)

a seasoned Getai army is awesome, and you can have so much variety...
personally I use mostly Draparai (sp?), Thraikioi peltasti, and horse-archers
shock infantry and phalanxes on the flanks to enclose and break the enemy, heavy peltasti to hold the line and give them hell, and cavalry archers to go around back and nail them with arrows...

Lusotanna are similar in their balanced armies and decent basic units, only thing being that they don't field the cavalry-archer types... not that their excellent charging cavalry doesn't make up for it.


but I say just try them all a lil and see which you like best