In our humble opinion, Sanity is the best and most flexible headless CMS out there, compared to other popular options like Contentful.
An introduction to "headless" content management
With a headless CMS like Sanity, the content of your business is stored as structured data, totally separate from the presentation. That means that when it's time to design and develop a brand new website, your content can live on and be presented in the new solution without starting from scratch. It also allows you to distribute your content to the web, emails, apps, anywhere.