(3) Blaise Pascal
If it's possible to be considered a genius and still be underrated, Pascal pulls off the trick. In the 17th century, Pascal made landmark contributions to religious philosophy (where he contended that God shouldn't be experienced through reason and logic), mathematics (where his development of Pascal's triangle laid the groundwork for the study of probability), engineering (he made one of the first mechanical calculators), and physics (where Pascal's law describes what happens to a liquid under pressure). Given all of these huge achievements, it's hard to believe he died at just 39.
"A-squared plus B-squared equals C-squared!" You can't even read his name without the equation running through your head, can you? Pythagoras was more than just a triangular one-trick pony, though. The monastic Pythagorean brotherhood he led conflated mysticism and religion with mathematics, a way of thinking that later influenced Plato. He postulated that individual musical notes could be translated into mathematical equations, a notion that helped form early music theory. All of these achievements are impressive, but really, never forget that he helped make "hypotenuse" part of your vocabulary.
Triangle time! We're going to go out on a limb here: this is the tightest contest between two mathematician/religious philosophers you'll see all week. You may scoff at Pythagoras now, but when you were doing those word problems about figuring out how long a ladder leaning next to a building was when you took the SAT, he was there for you. Likewise, during the countless times over the course of a day when you need to do a binomial expansion, you can rely on Pascal. Pascal was the child prodigy, while Pythagoras lived every day like it was a toga party. It's a tight race, but like a right triangle, one side's got to be the biggest. Which thinker's the bigger genius?
[See the whole bracket here.]