Leonhard Euler
Bernhard Riemann
Smartest living man is Ed Witten. The reason half the theorists working on string theory/M-theory are still working on it as opposed to LQD, Lisi's E8 model, and other various forms of QFT/GR unification is because Ed Witten is still working on it.
When some of the most intelligent people in the world say that the biggest reason String theory still looks like a possible TOE candidate is "Ed Witten believes in it."
There are too many to choose from to name a single person, but it sure as hell isn't some fuckstick artist or musician. I don't place too much faith in IQ tests either. Obviously, a person 15 standard deviations from the norm is going to be "smarter" than the average person, but beyond about 125 or so the tests just don't seem to provide any direct correlation between increase in IQ and increase in novel "brilliant" ideas that improve upon theoretical models of various fields of study.
MattT is probably pretty close to Ed Witten though.