Any MacOS pre- MacOSX, Excluding A/UX
Had the luxury of being able to back up your system with a option drag, so you could safely meddle with everything you were not supposed to.
ResEdit and MacsBug were quite useful, and the MacintoshToolbox was the API
*Note: It was not actually called Mac OS until System 7.6, prior to that release it was just known as System, i.e. System 5.1, System 7.1, etc…**