Actually I think focusing on such questions can be a not very useful exercise, but here I am asking it anyway. As it is, indeed the Peoples' Republic of China (PRC) is indeed Number One on a number of important grounds, although probably the bottom line is that the world is now dominated by a G2, the US and China, with it unclear which is Number One overall. What has happened is that up until quite recently there was no question: the US was Number One as it had been for a long time. That is not the case now.
Probably the most important fact lying behind PRC moving into a possible Number One position is that it indeed does have in real PPP terms the world's largest aggregate GDP, probably on the order of 30% higher than the US's, with this gap continuing to grow. Most people are unaware of this, and it is hidden by the fact that the US continues to be Number One on nominal GDP, which gets constantly reported in the US media with no commentary or recognition of the situation regarding PPP GDP. As it is this complicated situation signals the likely current quasi-equality, because indeed nominal GDP is important as it reflects the ability of a nation to assert itself globally. But PPP does show how much it is really producing. And, assuming current trends hold, PRC probably will surpass the US even on nominal GDP within the next several years, almost certainly before the end of the decade.
Another important matter is that sometime in the last few years China replaced the US as the world's leading financier of development. Indeed, the PRC has accepted this role in a coordinated plan of action, its Belt and Road Initiative, which has come under some criticism by some nations for various reasons, including that it is an effort to achieve a dominance over the nations involved in this. But whatever the truth or falsity of that, this initiative is indeed leading to large scale infrastructure expansion in many nations that will aid their future economic growth. The US is not remotely providing such aid, and also is not going to be doing so. This places China in a very important position regarding the world economy, a position once held by the US.
Of course there are some areas where the US is still ahead. One involves military. China's military is growing and expanding, and it probably has the ability to cause US forces more damage in a conflict than many might expect, such as the ability to knock out an aircraft carrier and compete seriously in cyber and space warfare. But the bottom line is that if there were a full-blown war, the US continues to maintain an overwhelming edge. But let us hope we do not have to see such a test, although such comparisons are obviously important.
Certainly there are many other areas where the US retains the edge, even as PRC is rising in many of them. So in the crucial area of scientific and intellectual developments there is rising competition, but the US continues to broadly have the lead, even as China is taking it in various areas, some of them quite important, such as developing solar energy technology.
Needless to say the last four years has seen the US shooting itself in the foot on all this during the presidency of Donald J. Trump. "America First" led to America Second or worse. Angering allies and simply removing the US from so much going on in the world and violating treaties, left China as the relatively responsible party at the global level, and while China has engaged in hostile actions towards some neighbors, at the global level it looks more responsible than has the US, although this latter may be changing with the change in administrations in the US.
Anyway, clearly these two nations are global level competitors, with a long run trend tending toward the advantage of China, at least as it seems now. But let us hope this relationship can be managed without actual war breaking out.