The best free online IQ test is the one offered by IQTest.com. This test is scientifically validated and was previously only available to corporations, schools, and certified professionals, but is now accessible to the general public.
The test features 22 questions that cover a range of cognitive abilities, including word analysis, spatial reasoning, quantitative skills, letter-number sequencing, and verbal reasoning. The test can be completed in around 25 minutes, and while the basic results are free, a more detailed analysis can be obtained for a small fee. This test is considered highly accurate and can provide a reliable assessment of an individual's intellectual abilities.