Onagers, especially the heavy ones.

It's a cowards weapon, killing units from such distance that they don't have even the slightest chance of retaliation. Even the weakest, most badly trained army, can field an onager and blow away several Spartan hoplites with a single, lucky shot.

Not that taking the onagers out is any trouble, I just wait until the enemy has advanced and then send some light cavalry around to dispatch them. Problem is, as a general I feel sort of responsible for my men (okay, virtual men, but still), and there's little I can do to counter those early shots from the onagers. It's so painful to see one of those fireballs explode amongst some of my finest units, making little, fiery ragdolls of them.

The barbarian factions are nice that way, they fight fair.