I was suprised to see Silverstripe is still a contender. I guess it all depends on what you mean by custom development. I still like cakephp when you're starting with a blank screen. WordPress is good if you're just doing theme tweaks and not straying too far from the idea of a basic CMS. Headless Wordpress can give you some flexibility.
I’d say it really depends on what you want to build. WordPress is great for a quick start and tons of plugins, but for fully custom solutions, people often go with Drupal or even Laravel with a CMS built on top. It all comes down to the balance between ready-made features and total coding freedom
WordPress is widely regarded as the CMS offering the best flexibility for custom development, thanks to its robust plugin ecosystem, extensive theme customization options, support for custom post types and taxonomies, and a powerful REST API that enables seamless integration with other applications. Its strong community, comprehensive documentation, and scalability make it ideal for both developers and non-technical users, allowing for highly tailored solutions whether for small websites or large enterprises.
Here's a fact for all of you: 43.5% of all websites on the Internet work on WordPress. And that's for a reason.