I guess I would choose an Eide Mar denari. No other coin so graphically represents an act that had such profound and long lasting an effect on western civilization as this coin. For those non-ancient coin geeks, its a silver coin with two daggers that Brutus struck bragging how he killed Julius Caesar. It led directly to Octavian taking control of the Roman world and becoming the first Roman Emperor. That one specific act ended the Roman Republic and ushered in the Roman Empire.
I am working on my ancient geekdom. I may change my mind as I learn more, but for now, I need to stick to something modern and within my limited knowledge.