Ancient or not - Agincourt !!!
just look at the scene - 6,000 dirty peasants with bows only against 20,000 knights (the best army in the Western world) , and a "swamp" in the middle .
the French don't know until now what fell on them that day
I can't really see a more unbalanced major battle in history and with such results and more important , with such effect on modern warfare