Scorpions and Heavy Onagers - but then, I'm playing a mod which increases the number of engines per unit. Currently, I have armies where most of the work is being done by massed artillery and a strong heavy cavalry wing. The infantry, poor beggars, are just there to provide defense for the artillery (and occasionally get hit by stray fire-pots for their efforts).

Scorpions for their range and accuracy, but many are needed to make a significant impact. Heavy Onagers for the range and sheer chaos they can cause.