[MilStdJunkie]

Asciidoc. Particularly if you 1) need XML interoperability, 2) complex print outputs, 3) complex tables, 4) transclusion (partial and otherwise) in core spec, 5) conditionals in core spec.

The AsciidocFX program is a good "starter's editor" for those unfamiliar with Asciidoc and lightweight markup in general - it includes a "boxed" DocBook-XSL pipeline as an alternative to the Ruby-based asciidoctor-pdf. For an actual production editor, Visual Studio Code with the Asciidoctor extension is very hard to beat. Github integration on top of VSC gives you some collaborative visibility, too.

On the PDF front, another interesting Asciidoc project is asciidoctor-web-pdf, which uses Paged.js and CSS to product extremely complex PDFs using web technologies (Chromium + Puppeteer, I think). That, asciidoctor-pdf (Ruby/Prawn), and DocBook-XSL are the main PDF pipelines.