I know it's not dialectical to say that capitalism "peaked" in a specific time or place BUT...
For me, it was the sinking of the Titanic. Everybody thought that baby was unsinkable. She was going to sail right through those ice fields without a problem. God himself couldn't sink that motherfucker. They were going to plow through all the way to New York, set a speed record, and make even more money for the White Star Line. Then the world decided to remind the bourgeoisie that sometimes, it pushes back HARD. Lots of poor people died, but a few rich pieces of shit went down with 'em.
Apparently after the Titanic sank, everything was different. WW1 broke out only a few years later. Capitalism began devouring itself from the inside, even though it's kind of always been doing that since it began. Maybe there is kind of a major quantitative shift, though, especially in the imperial core. Up until then, rich people usually benefited materially from capitalism. But the Titanic and the trenches in WW1 start to really give them a taste of what they had been doing to everyone else for such a long time. 1917 follows WW1. Then comes The Great Depression. Then WW2. Then the Cold War. In the year 1970, if you look at a world map of communist versus capitalist countries, it looks maybe like not the best situation for the capitalists. But neoliberalism has been saving their asses for the last fifty years. Godwilling this period of history is now coming to an end.
The sinking is also maybe an interesting study of ideology. When the ship struck the iceberg, only a few people knew that the situation was serious almost from the beginning: the first officers and captain up on the bridge, and the "stokers" or whatever working the furnaces at the bottom of the ship or wherever they were, since water was literally shooting in through the breached bulkheads and forcing them to leave or drown. People in the middle had no idea what the fuck was going on. Sometimes icy seawater would be literally swirling around their legs, and they would be like: "not a problem!" The ship's designer also knew that they were fucked within minutes of striking the iceberg. It took about two and a half hours for the ship to sink.
I'm reading A Night To Remember, a classic book full of survivors' tales from the Titanic, so that's why I'm thinking about this.
You are not allowed to say that capitalism hasn't peaked yet or that it's idealist for me to ask this question in the first place.
I'm also biased towards books I've read, which makes me think that the peak was 1991-2000. The Soviets were gone, the Chinese were liberalizing their economy, Cubans and North Koreans were suffering and every other socialist state was being put through restructuring by the IMF. The victory was so complete that in the west every person and institution in power became totally convinced that bourgeois democracy was the final form of human society, and the dot com boom would further cement the idea that technology would solve all of our problems.
The decline for capital in the west starts with 9/11, after which Americans either abandoned or failed to defend their supposed liberal principles. Meanwhile in the east Hu Jintao started the process of reigning in the Chinese National Bourgeoisie, proving that China never intended to reform itself into a liberal society after all. A few of the socialist states have been lost (ie Cambodia) and most of the non-ruling communist parties are now discredited socdems, but Cuba and the DPRK still stand.