YYYY-MM-DD. When I was working front-end at Nudge.ai, I found myself having to go to Moment's documentation to see how to format dates in various ways.
In order to make the process of date formatting faster, I created WTFormat.com, which gives you a date which you rewrite in your preferred format. The site then generates the Moment format code that would yield that output.
Rewritten in 2021 to support more formatting libraries, now using NuxtJS (Vue) and TypeScript.