Drupal is the open source content management system of choice. Drupal has excellent relational content-type (entity) management capabilities out of the box, and is very extensible while helping you maintain strong and cohesive brand identity through it's excellent granular access controls and separation of content and presentation.
I have experience with Drupal since Drupal 5 and have been actively developing Drupal sites since version 6. Since version 8, Drupal has adopted a object-oriented style with a complete rewrite, taking advantage of many Symfony components to create a highly modular enterprise ready solution.
Drupal 8 is ready to be decoupled, allowing an instance of Drupal to drive disparate interconnected interfaces such as apps, web sites, feeds and other API consumers. Drupal is designed to run an API that can drive desktop and mobile experiences seamlessly for the next generation of web apps.
At the time of writing, Drupal 9 is in the process of being released and for the first time, will be an in-place upgrade from Drupal 8. This is very exciting news and we will start testing this on our web properties soon.