There was a time when I would have said the American Civil War (thank you Mrs. Bokras, my 7th grade Soc. Studies teacher). But now, I think it would be the last 5-6 centuries B.C., world-wide. This is the period of time that gave rise to so many of the great philosophical and religious ideologies that still resonate today; the Greek philosophers, Judaism, Zoraster, Lau Tzu, Confucius, the Buddha, Jesus of Nazareth, and many more.
Honorable mention has to go to the period of the Enlightenment and the rise of the scientific method.
p.s. Thank you Lilrain for continuing to contribute here.
I was never a history fan therefore I can't think of a favorite part.