For me, org-mode is my go to markup language. So much so that I now find writing in Markdown to be almost painful. A few of my favourite features are:
- Tables are amazingly easy in org-mode, with spreadsheet features thrown in as well.
- Code snippets can be written in their own major mode, giving me syntax highlighting, auto completion and the ability to run the code and embed the output into the markup.
- Local links autocomplete, so I don't worry about breaking them with typos.
- I can export to HTML, Markdown, ODF or PDF.
- GitHub supports org-mode Readmes