Oh, I... Really can't decide. Every one of them is interesting. And they've all contributed to the long, eternal journey to the point where we're at now, and then beyond.
The ancient Egyptian civilizations are interesting in their way, the Babylonians in another, the mongol Khans, the Roman Empire, the Chinese dynasties and cultures, the norse, the middle ages, the enlightenment... I just can't decide. I think they're all exciting.
But if I have to choose, the First World War and the period beyond is probably the most exciting for me, considering what a titanic change it was, both political and cultural. Likely even greater than the Black Death or the industrialisation.
Of course, there's the future. Say what you like about it, but it's anything but boring.