What is your favorite Stack Overflow answer?

Two answers spring to mind. Both of them are very popular so it’s not just my opinion that they’re good.

First, RegEx match open tags except XHTML self-contained tags

This one, by bobince, is very well-known. It’s good because it manages to marry an infomative answer with something that’s very creatively-written. These sort of answers are generally frowned upon on Stack Overflow, but some of the old ones that were very popular are grandfathered in. The style of writing isn’t arbitrary and done for simple humour value, it’s done to reinforce why you should not attempt to do what the asker wanted.

Two answers spring to mind. Both of them are very popular so it’s not just my opinion that they’re good.

First, RegEx match open tags except XHTML self-contained tags

This one, by bobince, is very well-known. It’s good because it manages to marry an infomative answer with something that’s very creatively-written. These sort of answers are generally frowned upon on Stack Overflow, but some of the old ones that were very popular are grandfathered in. The style of writing isn’t arbitrary and done for simple humour value, it’s done to reinforce why you should not attempt to do what the asker wanted.

Second, What is your most productive shortcut with Vim?

This answer, by Jim Dennis, doesn’t just address the question as worded, but goes into great depth as to why the question was even asked.

Your problem with Vim is that you don't grok vi.

How do I install PHP 7 (PHP next generation) on Ubuntu

This is unabashedly self-promotion. I wanted to build PHP 7 from scratch and ran into all sorts of issues doing so. Eventually I got this to work. SO allows you to answer your own question — which I did (once I figured it all out) and this answer became one of the most popular answers for installing PHP from scratch on Linux — I’m very proud of this answer because of the work it took to figure it out and that it helped so many others.