How can one say who the greatest genius of all time might have been. Probably someone who died in infancy, or in a battlefield before he could demonstarte his genius. However, of those in history who we know something about, Isaac Newton gets my vote for whatever that might be worth.
Newton contemplated how the force of gravity worked simply by observation. Deduced it was an intrinsic property of matter, and then extrapolated the concept from terrestiral phenomena (i.e. and apple falling to the ground) to the celestrial, the orbiting of the moon and planets, the motions of the universe. That marvelous demonstration of intellect in and of itself is worthy of considering him the greatest genius. But there was a big problem. He needed a mathematical proof and the math of his day was not sufficient to prove his Law of Gravitation. So what did he do? Many years before Leibnitz (whom Newton accused for good reason as having plagorized his calculus) Newton set out to invent the math necessary to prove his ideas. Now how many people could do that! He invented a math that did not even exist. Not only did he revolutionaize our understanding of the universe and how it works, but he invented calculus just so he could prove his ideas. How many people could do that! Not even Eintsein did that. Einstein used existing mathematics (actually math that rooted back into the calculus Newton invented) to develop his General Theory of Relativity.
The only other historical figure who compares to Newton's genius is Archimedes. When Newton was in his 50s, a problem was circulated by Johan Bernoulli through Europe to solve, its called the Brachistochrome problem. After six months it was not solved by any of Europes greatest mathematicians. It was deliberately not sent to Newton the first time around. The time to solve the problem was extended another six months by Bernoulli. Over time the problem made it's way to Newton, the day before the second six month limit was due. It was said Newton had the problem solved in one night, and submitted the solution anonymously. Bournoulli proclaimed he knew the "lion by its claw." Newton was in his 50s. He had to again invent another branch of math to solve the brachistochrome problem. We now know this new math as the Calculus of the Variations. At age 54 Newton again invented a new branch of math to solve a problem overnight that no European mathmatician could solve in a year's time!