Markdown!
If by raw you mean a field called raw, then you’re looking at the actual Markdown source that we store. For an example, this is the JSON endpoint for you last post just now.
The raw field there is the actual text you composed in the Markdown editor, and we store it as-is so it doesn’t get more pure than that.
Instead, if you generally mean “the raw HTML” as scraped without using JSON endpoints, then you can turn that HTML into Markdown externally with pandoc as suggested above, or any other software.