commonmarker wraps comrak, a Rust port of GitHub’s cmark-gfm. It’s CommonMark- and GFM-compliant, fast, has many extensions and built-in syntax highlighting, and gives you an AST you can modify. Pick it for GitHub-compatible rendering.
kramdown is a pure Ruby parser with its own superset of Markdown (tables, footnotes, definition lists, attribute lists, math) and many output formats: HTML, LaTeX, kramdown, man pages. It’s Jekyll’s default. GFM input needs the kramdown-parser-gfm gem.
Redcarpet is a fast C extension based on the old Sundown parser. It isn’t CommonMark-compliant, but its renderer classes are easy to customize. Many older Rails apps use it; it gets few updates.
commonmarkercommonmarker is a CommonMark and GitHub Flavored Markdown parser for Ruby, built on the Rust comrak crate. How to convert Markdown to HTML, set options.
kramdownkramdown is a pure Ruby Markdown parser and converter used by Jekyll. How to convert Markdown to HTML, parse GFM, generate a table of contents.
RedcarpetRedcarpet is a fast Markdown parser for Ruby written in C. How to render Markdown to HTML with tables and fenced code, filter HTML, build a table of contents.