Nah, socialism is better and it has existed

Best, I think, carries a lot of loaded meaning. It's certainly very good at making stuff and concentrating stuff, but it also creates a lot of human misery

Capitalism tends to be a faster way of expanding the productive forces than feudalism (which functions via tightly controlling a well-defined region and periodically looting it) or slavery (which involves industrial scale kidnapping to replace an industrial labor force that is rapidly worked into exhaustion and death). That's because the capitalist system focuses on reproducing infrastructure improvements to improve production, rather than just robbing primitive locals or throwing generation after generation of kidnapped foreigners into the industrial meat grinder.

However, what we've seen both in the nascent socialist experiments in the USSR and the modern socialist experiment in China (and, to a lesser degree, social democracies in Europe and the US) are centrally managed economies that seek to balance out competing individualist interests while building out capital projects, such that universal higher quality of life is achieved. This grants the civilian population more opportunities to pursue advanced professional careers. And these professional careers add more unit-value to the overall economy than primitive agriculture or slave labor. What's more, because the economy generates a broader base of professional specialists than modern capitalist economies, the overall value-add of residents is compounded with population size. In early 20th-century capitalist societies, you were pulling from a talent pool of thousands of petite bourgeois households in England and tens of thousands in the US. Meanwhile, the USSR was mobilizing a competitive number of specialists despite a smaller-than-US population. China is mobilizing greater percentages while capitalizing on a larger population, promising the Chinese state as the forerunner of future professional developing on a world stage. India has the benefit of an enormous population, but is hindered by its caste system, such that it will likely eclipse its Western peers but not its large socialist neighbor.

TL;DR; Socialism is the most effective means of cultivating and rapidly reproducing a large professional labor force. From that labor force, you generate more productive capital improvements and reproduce them across your country more rapidly. And from that improved capital you generate more economic value, which opens up more opportunity to expand your professional labor force.

That is, fundamentally, what makes it the best economic system to date.

Incidentally, also why anarcho-whatevers tend to fumble the ball. They can't socially reproduce like strict hierarchical systems - be they feudal military dictatorships or fully communist command economies.