At work I selected GraphCMS to be our preferred headless CMS. It is already leveraged on multiple projects and has been working out well for us.
If you are not familiar, GraphCMS is a flexible headless CMS with native GraphQL support. It is not a REST API with GraphQL slapped on top, it is a true GraphQL service. This combined with their excellent schema editor and content tools makes it an excellent choice for headless content management.