Drupal
Trial
Drupal is an open source content management system and framework based on a PHP stack. It has a huge community, so it's no wonder it's among the top 10 CMS worldwide in terms of market share.
At AOE we consume Drupal mainly headless via JSON API. We appreciate its large feature set and mature plugin system as well as the general ecosystem.
In addition, the extensive documentation and setup with DDEV make it easy to get started.