the sweboz roster is even smaller yet about all of their units are very usefull, even the skutjans, second worsed archer but really usefull in the early sweboz campaign and thus ultra veteran later on^^

even tho there probably were myriads of these threads already, I'll join the fun:

Illyrian hippeis - second cheapest lance cavalry(aorsi riders afaik) availible as mercenaries in the hottest spot(ie most provinces and factions) of the map. good charge cool secondaries and great stamina. perfect unit to bolster/replace your cavalry on campaign especially for KH and Sweboz(and saba) or to train how to use cavalry in EB :D as they die otherwise^^.

runner up: Hoplitai haploi,Germanic swordsmen, Chatti youthlings, baltic frontiersmen, all horse archers with a lance, Parthian cataphracts, Early baktrian BGs, babylonian heavies, lusotann rastamen, Kinsmen cav several getai units and the utterly useless massilian hoplites(but they look cool and have a nice concept^^)