Although the rankings of national economies have changed considerably over time, the United States has maintained its top position since the Gilded Age, a time period in which its economy saw rapid expansion, surpassing the British Empire and Qing dynasty in aggregate output. Since China's transition to a market-based economy through privatisation and deregulation, the country has seen its ranking increase from ninth in 1978 to second to only the United States in 2016 as economic growth accelerated and its share of global nominal GDP surged from 2 % in 1980 to 15 % in 2016. India has also experienced a similar economic boom since the implementation of economic liberalisation in the early 1990s. When supranational entities are included, the European Union is the second largest economy in the world. It was the largest from 2004, when ten countries joined the union, to 2014, after which it was surpassed by the United States.

Who had the worlds largest economy in 2014? OK. Let's think. My stream of consciousness:
The relevant sentence in the passage is: Although the rankings of national economies have changed considerably over time, the United States has maintained its top position since the Gilded Age, a time period in which its economy saw rapid expansion, surpassing the British Empire and Qing dynasty in aggregate output. The answer is the United States.