'The ideas of economists and political philosophers, both when they are right and when they are wrong are more powerful than is commonly understood. Indeed, the world is ruled by little else. Practical men, who believe themselves to be quite exempt from any intellectual influences, are usually slaves of some defunct economist.' Keynes

'It is very easy to overestimate the importance of our own achievements in comparison with what we owe others.'

— attributed to Dietrich Bonhoeffer

An old post on the EA forum asked 'Who has done the most good?'. People often say Deng, as his economic reforms lifted hundreds of millions of Chinese out of poverty. True, Deng was important, but on the other hand, Deng propped up Mao and the actual economic policymaker, Zhao Ziyang, was written out of history for supporting the Tiananmen Square protests.

Indeed, in 1978, Deng told the vice premier to study the successful experiences of capitalist countries and bring them back to China, and then Chinese economists traveled the world and collaborated with foreign experts—including Nobel laureates, World Bank officials, Eastern European veterans, and free-market advocates—to find new economic strategies.^[1]

Some people stick out in terms of potential counterfactual impact:

E. Lim, World Bank's Lead Economist for China in the '80s^[2]
W. Brus, who was being listened to because he was socialist reformer^[3]
Tobin and Cairncross, Keynesians who gave lectures in China early on
Xue Muqiao, a Chinese economist, sold ~10M copies of his book in 1979 describing 'a sweeping, detailed survey of China's economy and the problems of its socialist system'.
Gregory Chow lectured on economics in China in the 80s. Speaking Mandarin, he became the primary academic liaison between the US and China (and designed an elaborate system to select top Chinese college students for US graduate studies in economics).
The Ford Foundation executives provided several hundred thousand dollars for exchanges with China during that time.^[4])

You could trace the impact further:

Neoliberalism: Chow did his PhD at University of Chicago in 1951, where he was influenced by Friedman, who was very much influenced by Hayek, who looms large as he shaped the neoliberal movement.^[5] His Chicago school, funded by the Ford and Rockefeller Foundations ( one grant officer at Rockefeller might have been particularly influential, ^[6],^[7]), influenced the Chicago Boys who contributed to the Miracle of Chile and de Soto, who helped inspire the Washington Consensus.^[8] (cf. Radicals for Capitalism^[9]).
Keynesianism: Keynes not only influenced many economists (like Tobin), but also invented the IMF and World Bank, which then transformed quite a few economies.^[10] Another example is Spain, which was poorer than Latin American countries in the 50s, and became rich because the IMF, World Bank, and OECD prescribed a rather standard stability plan: a program of fiscal and monetary stability, opening to international trade and foreign investment, and growing integration with international economic organizations.'^[11]

In several other countries, Economic advisers have often been suggested to have created growth:

Deming improved Japanese Manufacturing ^[12]
Economic policy advisor John Cowperthwaite has been called Hong Kong's 'architect of prosperity'^[13]
Albert Winsemius, a Dutch economist was the economic adviser to Singapore from 1961 to 1984. ^[14]
Western countries set up the Intergovernmental Group on Indonesia to ensure a long-term coordinated plan of aid for Indonesia in the '60s^[15]
US advisers encouraged Korea to expand its export capacity (in part because of growing US military involvement in Vietnam^[16]) and western advisors argued cutting the birthrate was essential for fast economic growth and modernization.^[17]

Marxism: In contrast, Marx is probably up there of having held back economies from developing and probably had the biggest impact on the world but in a negative way. Marx's communism held sway over half the planet around 1973. If China dominates for the next 200 years, Marx's significance will be cemented. Modern China is fundamentally a Marxist society, with the CCP officially viewing its ideology as an evolution of Marx's theory. This ideology governs 1.3 billion people and the world's second-largest economy. While the communist world is often seen as the past, China is likely enduring and arguably a distinct, intensely modern civilization, not merely 'Imperial Confucian China.' ^[18] More recently, the Fabian society might have had a negative impact: 'The Fabian Society transformed communism from a call for bloody revolution by labor activists into the concept of incremental democratic socialism, essentially inventing the modern welfare state and the 'champagne socialist' intellectual (e.g. members included very influential people like George Bernard Shaw, Muhammed Ali Jinnah, Jawaharlal Nehru, and Tony Blair). Fabian principles heavily influenced the economic policies of newly independent nations like India, Pakistan, Nigeria, Egypt, and Syria.^[19] India in particular had a very centralized planned economy and low growth for decades afterwards, which has kept hundreds of millions in poverty^[20] and so maybe the Fabian society has contributed to this (cf Why China got rich and India didn't).

Tracing this impact further: Western countries industrialized because of the influence of certain people as described in a SSC review of Global Economic History, which probably improved the educational outcomes of many of the above economists:^[21] By 1800, a developed/undeveloped gap emerged, with Britain as the only developed nation. Other Western countries, recognizing this, pioneered development economics, asking how nations industrialize faster. Britain industrialized accidentally, but thinkers like Friedrich List (Germany) and Alexander Hamilton (US) independently developed the Standard Development Model:

1. Create a large internal market by abolishing [internal] tariffs and improving transportation

2. Erect an external tariff to protect 'infant industries' from British competition

3. Create banks to stabilize the currency and provide businesses with capital

4. Establish mass education to speed the adoption and invention of technology

(Interestingly, this is eerily similar to an early Washington Consensus.^[22])

List and Hamilton were influenced by Adam Smith, where it's ultimately bottoming out. Of course, Smith also had influences, but he invented (development) economics, because Britain was the first to industrialize and he just described what happened. A survey of top economists shows near unanimous agreement that Adam Smith's metaphor of the invisible hand has been foundational to the development of modern economic theory, but has also been commonly misinterpreted as advocacy for pure laissez-faire capitalism.^[23]

Other contenders

Paul Ehrlich's 'Population bomb' partly caused China's one child policy, which had a massive [negative] effect.^[24] It was commissioned by the Sierra Club.^[25]
Stanley Fischer: Larry Summers said: 'No one had more cumulative influence on the macroeconomic policy makers of the last generation than Stanley Fischer. Central Bank governors Ben Bernanke, Mario Draghi and Kazuo Ueda were his students, as were many others like me, who served in other capacities. We all were shaped'^[26]
Karl Haushofer’s Influence on Hitler from Hal Brand’s ‘The Eurasian Century’:

Foundations: The term "geopolitics" was coined in the late 19th/early 20th century by Swedish intellectual Rudolf Kjellén and German geographer Friedrich Ratzel. Influenced by social Darwinism, these thinkers viewed nations as "living organisms" defined by racial terms that had to expand or die. This school of thought prioritized the coercive pursuit of lebensraum (living space), resulting in a form of geopolitics described as "vindictive and expansionist".
General Karl Haushofer, a WWI artillery commander turned academic, led the "Munich school" of geopolitics. By the late 1920s, he amplified his expansionist message through monthly radio broadcasts and a magazine that sold up to 500,000 copies annually.
Strategic Vision: Haushofer advocated for a "pan-region" strategy where Germany carved out an autarkic empire across Europe and Africa. He envisioned an autocratic alliance between Germany, the Soviet Union, and Japan to break British dominance.

The Connection: While his influence is sometimes exaggerated, Haushofer was indisputably present at the creation of Nazi geopolitics via his former aide, Rudolf Hess. Haushofer visited Hitler in Landsberg prison after the 1923 Beer Hall Putsch to teach him geopolitical theory.
Intellectual Legitimacy: Haushofer provided a "coherent explanation" for Germany's expansion, helping legitimize the regime's revolutionary designs.
Impact on Mein Kampf: Historians note that central arguments in Hitler’s Mein Kampf—specifically regarding the "illogic" of borders and the necessity of eastern lebensraum—were "pure Haushofer".
Inverting Mackinder: Haushofer heavily borrowed from British geographer Halford Mackinder, essentially inverting Mackinder’s warnings about a continental hegemon into a blueprint for German domination.