There is no simple answer to who has
the highest IQ in the world.

Marilyn vos Savant is one of the most commonly cited names because Guinness previously listed her with an IQ of 228. William James Sidis is often associated with an estimated IQ of 250–300, but this figure is not based on a documented standardized test result.
Terence Tao is another frequently mentioned name because of his extraordinary mathematical achievements, although commonly quoted IQ figures for him are estimates rather than a universally verified modern score.
The important point is that extremely high IQ scores should be treated carefully because different testing systems are not directly comparable.
More information:
https://thecampusreview.com/stem/highest-iq-in-the-world/