Query Monitor is a debugging plugin for anyone who develops with WordPress. It has some advanced features not available in other debugging plugins, including debugging of REST API requests, AJAX calls, the template hierarchy, script dependencies, redirects, and the ability to narrow down its output by plugin or theme.
Query Monitor has a strong focus on presenting its information in a usable manner instead of simply dumping out data. For example, aggregate database query information is made available, allowing you to quickly determine poorly performing functions, plugins, or themes.
Query Monitor is free and open source software, just like WordPress.