XML for machine-readable data because I live to cause chaos
Either markdown or Org for human-readable text-only documents. MS Office formats and the way they are handled have been a mess since the 2007 -x versions were introduced, and those and Open Document formats are way too bloated for when you only want to share a presentable text file.
While we're at it, standardize the fucking markdown syntax! I still have nightmares about Reddit's degenerate four-space-indent code blocks.
Markdown, CommonMark, .rst formats are good for printing basic rich text for technical documentation and so on, when text styling is made by an external application and you don't care about reproducible layout.
But you also want to make custom styles (font size, text alignment, colours), page layout (paper format, margin size, etc.) and make sure your document is reproducible across multiple processing applications, that the layout doesn't break, authoring tools, maybe even some version control, etc. This is when it strikes you bad.