At this point I really think GitHub should formally publish their flavor as well as a default implementation. It's likely the single most widely used variant online at this point.

I know there are others and there are fine points. I would like to see a couple minor additions to support image placement (that aligns with Medium's editor) and finally a strike-through text notation. But that's about it.

Github does publish their spec: https://github.github.com/gfm/ The CommonMark spec is largely based on it.