Discoverer of gravity, maker of calculus, and the genius who brought you F = ma, there is no other person who has so successfully pimp slapped mother nature into revealing her secrets ever since. If science was a sandwich then he was the angry drunk at home who beat mother nature into making him meat loaded science subs with every topping, roasted to maturity.
As pointed out in Mass Effect 2 he is also the deadliest son of a bitch in space, mainly due to his laws of motion and how they apply in absence of an atmosphere.
No matter how you cut it when people look to define the word genius, they are forced (in Newtons, of course) to admit that this sickly farmer in the Renaissance is the single best example of the word to ever walk the earth.