What year did the USA peak?
You can use economic measures, social measures, cultural, whatever you want.
I get good vibes from feel good 80s movies and also acknowledge that many cinema franchises were created that decade, and lots of iconic American pop stars, so I'ma say 1988, before the 90s arrived.
Ronald Reagan won all but one state when reelected so there wasnt a strong political divide also like today. Clinton was very polarising so the 90s basically is when that started.
But if you feel that the 60s was the peak, say that.
Or the start of the internet era in the mid to late 90s, say that.
You might even think it peaked in Obama's years. A sweet spot for some.