There is a saying that the work of ordinary geniuses can be reproduced while the work of exceptional geniuses--magicians--is so dazzling that you can't imagine doing it. Scott calls people who popularize science to their generations "prophets." Scott is a prophet and a magician and this is why he lands on the page of the smartest living people.
A cognitive scientist, philosopher and linguist, Steven Pinker has perhaps done more to move these important social sciences into the fold of the hard sciences than any of his peers. While not infallible, he is an exacting, insightful thinker, and for this reason he earns a spot on my list of the smartest people alive.
Ed Witten almost became an economist. Thank god he didn't, because physics and mathematics wouldn't be the same without him. With a Feynmanesque explanatory ability and a Penrosian mathematical prowess, this scientist takes names and kicks butt. He fits snugly on my list of the smartest people alive.
The irascible Murray Gell-Mann knows so much about so many things that it's hard to exclude him from this list, even if his legendary cantankerousness is not an admirable intellectual trait. Fundamental physics wouldn't be the same without him, and this list of the smartest living people wouldn't either.
Terence Tao. To some this name is synonymous with mathematical genius, to others with prodigiousness, and to some with glory. He coauthored a a beautiful proof that there are arbitrarily long arithmetic progressions in the primes, and maintains an extremely popular blog. He is an easy addition to my list of the top ten smartest living people.
L The List