When you take purchase power parity (PPP), then China is no1.
USA is 1/6 of world PPP GDP.
Why I believe PPP is more important than nominal?
A barber makes 60usd/h in USA, but only 20usd/h in China.
This is because Americans are richer, and everybody can charge more.
Does it mean that American barber adds to the economy 3 times as much as Chinese one? No, he simply charges more for the same service as Chinese one.
So USA numbers are inflated, and PPP tries to make it proper.