This question is difficult to answer.If we look at it relatively, two historical countries are a possible answer. The ancient Persian Empire among the Achaemenids, or the Tang dynasty of China.
The Tang dynasty lasted 289 years (from 618 to 907), but the economic decline occurred in the year 755, during the An Lushan Uprising, or perhaps earlier in 751.The actual duration of the economic culmination lasted about 137 years or 133 years long. Before that, the dynasty estimated 50% of the entire gross national Product of the entire world. So great was China’s economy during the Tang dynasty.
The ancient Persian Empire under Achaemenids lasted for 214 years (from 550 BC.Until 336 BC). The economic decay was already around the year 420 BC. Under Artaxerxes. The actual duration of the economic culmination lasted about 130 years. During this high point, the Empire had about 50% of the world’s population within the boundaries of its empire. As far as I know, GDP is not estimated. An estimation of the Sassanids is known, namely 37%, but that is five centuries later and they have had a short flowering time.
This would allow the Tang dynasty to come first, but it is not that simple.
First, the Tang dynasty really reached its flowering period around the year 642, when the G枚kt眉rken were defeated, while the flowering period under the Achaemenids started right at the start.The defeat of the g枚kturks meant that the Silk Road could be kept open in a stable way, shortening the actual duration of economic flowering to 113 years or 109 years and that is shorter than 130 years.
The second problem is that gross national Product is an approximate size and purchasing power does not say anything.Typical also to gross national Product is that it is expressed in money. The special thing about the old Persian Empire is that it has conquered countries where money has been created as a means of payment and has also been invented in a sense (although it is disputed), as for example Lydia.
A third problem is that population taking as a benchmark for the largest economy is also not exactly a good way to determine the size of an economy.You can have 50% of the world’s population, but if everyone in that country is poor, you can question whether it is the biggest economy. That is also a reason why gross national Product per capita was invented as a yardstick, because it also says something about how wealthy a population is.
The British Empire (perhaps third place on this list) is a good example of how this can work out.During its flowering period from 1870 to 1913, the British Empire had about 43% of global GDP, but all in all, only about 23% of the world’s population.
The United States is also a wealthy country and although the U.S. has had a flowering period during its ‘ Gilded Age ‘, the U.S. has only become the world’s largest economy after decolonization after the Second World War.The country currently owns just over 20% of global GDP, so historically certainly not the largest and not for the longest time.
Some people may also expect the Roman Empire to answer, but the Roman Empire has only a quarter of the world’s population at its height, and it is doubtful whether at all ever for some period the largest Economy.
There have been other empires throughout world history, but those were either short-lived, or not so rich.
All in all, it is a matter of saying that the answer is the Old Persian Empire among the Achaemenids, because economy in the broadest sense not only beats money, but on distribution, trade, production and consumption of goods and services.The old-Persian Empire was agrarian strongly developed for that time and knew a lot of trade.