Who is the smartest person you've ever known and why?

Quite a few people. I've been into many high-IQ circles and you'll find people with photographic memory, or that are absolute Leonardo da Vinci's in both engineering and art, meanwhile it's all impressing they all ended up with a similar destiny as me which is upper middle class. Really smart people have very good intuition and this prevents them from taking risks where the odds are extremely against them, like quitting Harvard to make a social network that ends up becoming a trillion dollar business.

They also avoid doing fringe research in academia, making them not typically stand out when they happen to have researched the new hot topic.

I've known also a few people that either founded $100B+ public companies or did research like people like to talk about in HN "All you need is attention" and they weren't the smartest, but smart enough, but were quite lucky on their journeys and to be in the right place and in the right time

He certainly didn't think of himself this way, but Ward Christensen was the smartest person I knew personally. He was always experimenting, trying to find new ways to do things.

The smartest person I've ever met, according to others, was Earl Pace[1], a retired Westinghouse engineer who designed and implemented the control systems for the 6 stand cold reduction mill at US Steel in Gary, Indiana. Pulling steel like taffy with huge motors, and a compound system involving DC generators as huge operational amplifiers strained the absolute limits of what could be done with 1964 technology, yet it ran for decades.

[1] https://www.chicagotribune.com/obituaries/earl-c-pace-in/

Why did I know them? Or why do I think of them as extraordinarily smart?

I think of "smart" as highly situational. I've known people with genius math or programming skills who can't change a tire or follow a recipe for making pasta.

I honestly can't answer that question, because it depends on context. People who are very smart in one sort of setting can be idiots in another, and vice-versa.

In my lifetime, I've gotten to know only two people who were unambiguous geniuses (although being a genius is a different thing than being smart). They were both broken people otherwise, and they led me to think that if that's the price of genius, then I'm very glad that I'm not one.

My dad, because he's overflowing with wisdom with just enough smarts.

My little one, he blows my mind each time I hear his reasoning and mature perspective on a topic that I would otherwise assume he would have no clue about!

the smartest person I ever knew was my first manager, would talk about SmartOS a lot, didn't even know about Solaris at the time so it was like magic to me

Me, because I said so.