Comment by II2II

It would also depend upon what you are trying to accomplish. You have simple filesystems and complex filesystems. You have simple video drivers, and you have complex video drivers. Simplicity gets the job done, but complexity may offer better reliability or performance or features.

Then there is the question of what one means by an operating system. While I'm sure that most people would agree that much of the software shipped with Windows, Mac OS, or the typical Linux distribution isn't part of an operating system proper, few would agree upon where the boundary lies.