Oh i think that the Early Legionary Cohort is the most cost effective, it's fairly cheap, and five of them with fire at will can beat most units, and you have enough money to make several armies of them.

Or the Armoured Hopliltes of greece, even Romans break when they are ordered to charge strait in against them. Plus they aren't as expensive as teh Spartan's and are almost as gd as they are.