I prefer 1 General, 4 units of Roman/Legionary Cavalry, at least 4 (preferably 6) units of Archer Auxilia and all the others (so between 11 or 9 units) Legionary Cohorts

I generally think the other Auxilia units are hardly worth building (maybe Light Auxilia when facing Elephants), and further you should always get any nice mercenaries that are around (Bastarnae, Elephants, Sarmatians, Cretan Archers)