Cahokia at its peak. I'd just walk around and record as much as I possibly can. There is an enormous knowledge gap about pre-Columbian North America. With an hour of recordings of people going about their day, we should be able to learn an enormous amount.

Any other times I would consider would similarly be about filling in knowledge we don't have, so someplace like Catalhayuk or Indus Valley/Harappan Civilization.

I'd say the Cretaceous or something but I think I'd be unlikely to see as much useful info, because wild spaces are often sparse. I might barely see any dinosaurs at all, and I'd have a way higher chance of dying.