Which king ruled the greatest kingdom as measured by fraction of global population?

George V of the United Kingdom of Britain and Ireland.

At the height of the British Empire, his government had dominion over more than 500 million people, out of an estimated global population of 1.5 billion.

No one else even comes close. All the crazy numbers thrown about for Rome or China are pure nonsense, not that anyone will stomach it today without screaming blue murder. There's no way in Hell Trajan or Taizong ruled over 30% of the World Population or 50% or whatever crazy number everyone in the big colleges seems to be pulling out of their arses.

44% for Persia. Seriously?

Anyway, it's a moot point. The West and China are true Empires and demand a history worthy of their imperial legacy- coupled with whatever underlying steppe psychoses go along with it in case of the former, no matter how absurd it may sound and the average guy is low IQ enough to swallow it.