The dominate, rigorously researched, and documented answer is German polymath Johann von Goethe (IQ = 210), second to Shakespeare in literature, with a vocabulary of over 90,000 words, inspiration to Darwin, with his theories on maxilla bone evolution, mental compatriot to Newton, with his theory of colors, and founder of the science of human chemistry, with his 1809 treatise Elective Affinities, wherein a human chemical reaction view of life is presented, some two-hundred years ahead of its time.
Other individuals, to note, can be found to have had stated IQs above 210 (either verbalized, e.g. Leonardo da Vinci (IQ = 225) or William Sidis (IQ = 250-300), or based on childhood ratios, e.g. Michael Kearney (IQ = 325) or Marilyn vos Savant (IQ = 225)), but these values are generally found to be over-estimates, based on oversimplification, when compared to that person’s actual adult IQ or when each person is fitted into a robust comparative historical study (collective of 300 geniuses or more).
To give an example, at age four, American Michael Kearney scored 168-plus on an IQ test for six-year-olds and via mathematical juggling, using his age and test score, he was said to have what is called a “ratio-IQ” of 325. Twenty years later, however, although he turned out to be a relatively smart individual, completing a BS in anthropology (age 10), MS in biochemistry (age 14), and MS in computer science (age 17), he had difficulty getting past the half-way point on the pop-intelligence game show Who Wants to be a Millionaire?, leaving with only $25,000, implying that ratio or estimated IQs are not as accurate as historically-determined IQs.
● IQ: 200+ (Encyclopedia of Human Thermodynamics)
● Cox, Catharine, M. (1926). Early Mental Traits of Three Hundred Geniuses (Genetic Studies of Genius Series), Stanford Univ Press.
● Kearney, Kevin J. and Kearney Cassidy Y. (1998). Accidental Genius. Woodshed Press.