I've thought about this question many times.

It's hard to narrow it down to just one.
If I had to pick a time where I could travel back in time and be a fly on the wall there are a few times and places that come to mind:
First without a doubt would be the time of the Siddhārtha Gautama (Buddha) I would like to have been there to see him and listen to him talk about life and the nature of reality.
Second would be the time of Jesus for the same reasons. I would have liked to hear what he actually said instead of hearing it from second hand sources.
Enter Metallica Song "Creeping Death": Time of Moses. How the hell did Moses get the Pharaoh of Egypt to release the Hebrew Slaves? Mindblowing. I'd would have liked to be present to see Moses and what in fact transpired with the parting of the Red Sea!
Medevil Europe. Ancient China...
But if I had to choose just one I think the future to see what becomes of mankind. Do we make contact with other intelligent life? Do we destroy ourselves and the earth?
Do we reach the technology where we can travel to other galaxies/solar systems and keep life going?