IQ is the strongest predictor of:

Job performance https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4557354/

Occupation, Education and Income level https://emilkirkegaard.dk/en/wp-content/uploads/Intelligence-and-socioeconomic-success-A-meta-analytic-review-of-longitudinal-research.pdf

From AltHype:

We know that IQ tests measure intelligence because IQ tests correlate with peer and self rated intelligence. For instance, in Denissen et al. 2011 489 college students were divided into 20 groups which studied together for a period of one year. At the end of this year, subjects were asked to rate how intelligent their group mates were on a 7 point scale ranging from “not intelligence” to “very intelligent”. It was found that the better a subject did on an IQ test the smarter their group mates thought they were.

Palhusand Morgan 1997 found similar results and also showed that the correlation between peer rated intelligence and IQ increased the longer the peer knew the person being tested. They had 5 group discussion sections, and in the first section, found that in the first session, intelligence ratings were almost entirely a function of how much people talked. By the Fifth session, it was almost entirely a function of the person’s IQ – so talking a lot only increases perceived “intelligence” above IQ in the short term.

Similarly,Bailey and Hatch 1979 showed that intelligence rated by people’s close friends correlated with their IQ and Bailey and Mattetal 1977 found the same was true of spouses.

A significant body of research has also shown that IQ tests predict how intelligent people rate themselves as being (Paulhus, Lysy, and Yik 1998, ,Angelo and James 1977, and Reilly and Mulhern 1995). Clearly then, the smarter a person thinks they are, and the smarter their friends think they are, the better they tend to do on IQ tests.

So in summary, IQ tests predict life outcomes better than several factors commonly recognized to predict life outcomes, such as what your parents are like and how good your grades are. And IQ predicts one’s subjective perception of a person’s intelligence the longer you interact with them.