Pick on the constraint that will not move, because everything else is negotiable and this decision is not easily reversed.
Your editors must work without a developer. Storyblok. The visual editor is the product rather than a feature bolted alongside one, and no other platform here matches it for a marketing team that ships pages weekly.
You are building on Next.js and want one repository. Payload. The content model, the admin panel and the API are part of your application rather than a service beside it, and the generated types mean the front end knows every field name.
You want the content model reviewed in pull requests. Sanity, Payload, Strapi, Statamic or Prismic. On all five the model is files, so a schema change arrives as a diff rather than as something a colleague did in an admin panel on Tuesday.
You are shipping marketing pages fast with a small team. Prismic. The slice library keeps editors inside components you control, the pricing ladder has four published steps rather than a cliff, and pulling the model generates your types on the way through.
You have a database already in production. Directus. It sits on top of what you have rather than asking you to move content into a second store, which no other platform here offers.
You are publishing into several markets and languages at once. Contentful. Multi-space and multi-locale delivery is what it is genuinely best at, and it is the reason large organisations keep choosing it despite the cost.
You want to own the whole stack and extend the CMS itself. Strapi. Community edition is MIT, the plugin ecosystem is the largest here, and cloud pricing is per project, so adding editors costs nothing.
Several systems need to sit behind one API. Hygraph. Content federation pulls other services into the same graph, which nothing else in this list does natively.
You build many sites on Laravel. Statamic. A one-time licence per site rather than nine subscriptions changes the economics for an agency, though the headless APIs need the Pro tier.
If none of those constraints is binding, the honest answer is that Sanity, Payload and Storyblok will all serve a team of five to fifteen well, and the deciding factor will be whether your editors need to see the page while they work.