MarkEdit and Downer are both free, both feel at home on a Mac, and both work on plain Markdown files. They’re built for different halves of the job.
MarkEdit is a lean editor
MarkEdit describes itself as TextEdit for Markdown. It’s a small download, opens large files with ease and sticks closely to GitHub’s Markdown rules. You get multiple cursors, code folding, macOS Writing Tools, Shortcuts and AppleScript. Preview, themes and more come as official extensions, and the preview extension can draw math and Mermaid diagrams. Its source is published under the MIT licence, and it says it doesn’t collect any user data.
Downer is a reader first
Downer opens every file as a finished page, almost instantly. Long documents get a map of headings and a command palette, and Mermaid diagrams, math and LaTeX papers are drawn without extensions. Files keeps your recent documents with previews, and opening a folder gathers its Markdown into a collection. Quick Look previews in Finder come with it. When you need to change something, the editor slides in beside the preview.
The short version
If you write most of the day and want the lightest possible editor, choose MarkEdit. If you open a lot of Markdown, papers and diagrams and want them looking their best, choose Downer.