Gottfried Leibniz should make the short list for any discussion of true genius. His insights into mathematics helped shape our understanding of Calculus and Physics, and he also wrote extensively about probability, biology, medicine, psychology, linguistics and logic. He articulated differential and integral calculus at the same that Newton was doing it, and we even use his notation in modern calculus.
Leibniz was also fascinated by philosophy, because his mind wasn't only full of numbers and formulas, but truly astonishing ideas of which his math was only a byproduct. Nowhere is that more clear than in his Philosophical Essays and Texts, especially the two most famous of them, Discourse on Metaphysics and his infamous Monadologie.
Historically, Leibniz was a contemporary of important philosophs like Spinoza and Voltaire. This meant that his philosophies never really reached as great a height as his astounding calculus and scientific advancements. In fact, within a few years of his death, Voltaire parodied his Théodicée. However, since his death, the true genius of his mathematical insights led academicians to reconsider his arguments, most famously, the Monadologie which essentially argued for a metaphysical realm underpinning this one, in which all matter, organic or not, was a biological superbeing that expressed itself as an infinite amount of an infinitely small spirit being called a Monad. These Monads may remind current readers of modern physics, but he wasn't thinking of physical science exactly when he wrote the work.
Leibniz is basically still a mystery since his works didn't have the cultural sway of some of his contemporaries, and since his ideas are bizarre at best. These issues are compounded by the fact that Leibniz's philosophical works came into vogue after his death, before he could clarify his more adventurous ideas.
