Ancient: Elephants, you can discover Polytheism very quickly, and while the other tribes think they're very smart for learning Bronze working they forget that an Elephant can trample a Phalanx with ease. I use hordes of elephants to punish my nearby opponents early on.
Renaissance/Industrial: The Cavalry. Once again, the untoppable offensive force against both Musketeers and Riflemen.
Modern: The Spy. Wrecks havoc in my opponents cities. Undermines their democracies, poisons their water, incites the dissidents...
I've also got to recognize Diplomats, who sacrafice themselves taking out city walls until the spies are available.
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CoH: Back, after a long hibernation with many fat, lady hippos and daily donut delivery.