Which famous scientist developed the theory of gravity after seeing an apple fall?

Isaac Newton, and the apple story is better evidence than its reputation suggests - he told it himself, late in life, to several people including William Stukeley, who wrote it down in 1752. It was never a blow on the head. The insight was the question it prompted: why does the apple fall straight down towards the centre of the Earth, and does whatever pulls it also reach as far as the Moon?

That the answer is yes is the whole revolution. Before Newton, the heavens and the Earth ran on different rules; afterwards, the same equation governs an apple, a cannonball and a planet, and the Moon is simply falling continuously and missing. It was the first demonstration that laws found in a garden apply to the universe.

He did much of it in 1665 and 1666, sent home from Cambridge because the plague had closed it - the same eighteen months in which he also did foundational work on calculus and on optics, and which are usually called his year of wonders. He was in his early twenties, and then largely sat on the results for two decades until Halley badgered him into publishing.