Which are your favourite barbarians and why...
Swêboz. I like their units most, for design reasons as well as for the badass factor (not to forget their funny names). OTOH, their economy is just pathetic, which makes a Swêboz campaign tougher as times proceed and the "civilized" competition, especially those nasty Romans, becomes increasingly capable of outproducing you.
Casse are cool, too. Although it is a pity that chariots become unavailable with the second reform.

Basically, if you want lots of spearmen (every "native" Germanic unit except for the Druxtiz Basterniska carries either a spear or javelins or both!), AP and frightening units, Swêboz are ideal for you.
If you want to have the best selection of swordsmen in the game plus a decent economy (paved roads etc.), play Aedui or Arverni.
If you want to play hide and seek on the battlefield, the Lusotannan are first choice (with Swêboz and Getai being second). Have fun with your cavalry hiding in long grass. Plus, every Iberian unit except for slingers and melee cavalry carries javelins.
If you want a really diverse unit roster, play Getai.
If you want to go "crazy", play Casse.