Best location for a debugging script

Maybe as a must-use plugin?

From docs,

  • Always-on, no need to enable via admin and users cannot disable by accident.
  • Can be enabled simply by uploading file to the mu-plugins directory, without having to log-in.
  • Loaded by PHP, in alphabetical order, before normal plugins, meaning API hooks added in an mu-plugin apply to all other plugins
    even if they run hooked-functions in the global namespace.