I guess this is technically stack exchange, not stack overflow, but the answer to this math question is amazing:

https://math.stackexchange.com/questions/1749853/if-there-ar...

That's brilliant. So intuitive.

Probably this one on sorting socks. It perfectly displays the stack overflow community and the silly seriousness developers seem to have:

https://stackoverflow.com/questions/14415881/how-to-pair-soc...

Do you mean other than the famous "RegEx match open tags except XHTML self-contained tags" [1]?

[1] https://stackoverflow.com/questions/1732348/regex-match-open...

Great answer but it depends on the use case. If the regex is for manually searching through a "view source" then who cares if finds the odd false positive?

Probably this one: "Cycles in family tree software" https://stackoverflow.com/questions/6163683/cycles-in-family...

I like this one

"What is the best comment in source code you have ever encountered?" https://stackoverflow.com/questions/184618/what-is-the-best-...

I recently found the following, and it just blew me away:

https://stackoverflow.com/questions/8318911/why-does-html-th...

You can also run queries and find which questions are most upvoted.

http://data.stackexchange.com