Who is the 'smartest' human being?

Salman Khan.

In the year 1998 at MIT, Khan achieved three BSc in mathematics, electrical engineering, and computer science whereafter he completed two MSc in electrical engineering and computer science. He also holds an M.B.A. from Harvard.

He left his job as a hedge fund analyst after discovering educating millions of people—through his paragon teaching style—was in the realm of possibilities.

I believe the smartest people who roam the universe are simultaneously the most unpretentious because through hindsight they realize it is not inherent intelligence, but persistent passionate diligence, curiosity and a willingness to grow that's key. They also took notice that the environment where kids grow up, which makes or breaks passion, is entirely up to chance; and an unfavorable one at that.

That being said, Sal chose sharing wisdom over utilizing it for lucrative intentions. And judging from his various degrees, the latter possibilities could've been endless. To this day Khan Academy has offered a helping hand to over 40 million people worldwide, for free!

"The sole meaning of life is to serve humanity." ―Leo Tolstoy

His genuinely compassionate and charismatic approach has impacted and will continue to impact millions of lives. Team Khan Academy is closing the illusionary gap between the layman and the wonderful Shangri-La that is science. Ultimately leveling up our humanity.

Sapere aude!

Footnotes
[1] Khan Academy
[2] Salman Khan
[3] Rethinking Education - Sal Khan

PS The claim that any person is the smartest who ever lived is subjective. Sal was chosen because he educated people who'd otherwise never have enjoyed the opportunity. By virtue of making the humankind smarter he deserves an honorable mention. Khan Academy might have tutored the next Marie Curie or Albert Einstein who'd in different circumstances be left in darkness.