Well I find Carthaginian longshielders to be the best overall. You get them really early in the game and unless you do something relly stupid like run them thru a phalanx they can beat everything. I had one longsh unit (silver upgrades) destroying one praetorian and one auxilia cav at the same time and losing only 19 men. The whole battle was a "heroic victory" my 607+164 reinforcements (town militia, lybian spears, iberians and 1 unit of numidians) carthaginians slaughtered a 1624-men strong Julii army killing over 1200 and losing 400 men. It took me almost an hour paying and suffering. But my Sacred Band solved the situation really well. That was their test battle.