Probably Strapi https://strapi.io/

I'd probably go with one of these:

- For clients: directus.io

- For internal tools: pocketbase.io

- For markdown maximalists: tina.io

Not headless in the classical sence, but I would be stupid to not mention my own foss CMS here: nitropage.org

Nitropage's focus is visual page building, great for website content, not ideal as an app backend. With the (p)npm create installation approach you are able to develop your own blueprints (building blocks) via SolidJS components.