I'd say it varies. Conservatives will reminisce the Reagan era....moderates will reminisce the Clinton era... progressives the Obama era
ETA: many (most?) will agree post 9/11 was a unifying time (for a quick minute)
but I'm sure all sane people will agree that, currently, our country is not in a good place.
I don't think it's about going back to a specific time, it is about the direction the country is going in.
As I understand it, MAGA is a response to the feeling that the country is going downhill, even if for now we are still better off than most of our history.