Gianni Pudic posted this question on 21st July 2020 on Quora.com. His answer ranked (his words) the raw ability of the person and not necessarily to the contributions they made.
This child was able to read the New York Times at only 18 months old. At age 8, the boy was fluent in eight languages (Latin, Greek, French, Russian, German, Hebrew, Turkish, and Armenian). At the age of 11, he was finally enrolled in Harvard and gave lectures in higher mathematics. His giftedness was evidently off charts, so much so that MIT physics professor Daniel. F. Comstock claimed that this individual’s talent was matched only by one historical figure, namely by that of German mathematician Carl Friedrich Gauss.
#2 Carl Friedrich Gauss (1777–1855)
Carl Friedrich Gauss said he was calculating before he learned to speak properly. He is most famous for solving the assignment of his math-teacher, which was to add all numbers from 1 to 100 (1+2+3… +98+99+100). According to French mathematician Laplace, Gauss was the best mathematician to ever exist and that by a large margin.
#3 John von Neumann (1903–1957)
If you were to rank them by word of mouth however, this man would easily take the number one spot on this list. He was an allrounder, knowledgeable in physics, engineering, maths, and computer science. He was said to be able to recall any sentence (word for word) of all the books he read – plain simple perfect recall.
#4 Sir Isaac Newton (1643–1727)
According to the wishes of his parents, Newton should have become a farmer but didn’t meet their wishes. Instead, Newton dedicated his life to algebra, mechanics, and optics. He also single-handedly invented calculus. This alone provides you with have a rough estimate of what this genius was capable of. Many regard Newton as the most influential scientist of all time.
#5 Nikola Tesla (1856–1943)
Nikola Tesla, the man who was ahead of his time, was born in Serbia. He became a physicist and electrical engineer is most famous for being the inventor of the alternating-current (AC) system, which today is the prevalent electrical system across the world. In 1926, Tesla even predicted wireless data exchange. Bizarrely, because of this and many other visions, he was declared as a mad man and not too seldom, also a dreamer. As for his cognitive abilities, Tesla was able to perform integral calculus in his head to the extent that teachers at the time believed he was cheating.