Let us make honorable mentions here for some that did not make the shortlist:
1. Red Star OS. North Korean Linux based macOS lookalike. Not sure if you should be happy or sad if you get to use it. 
2. Windows Phone 10. Lets all just remember Microsoft's past CEO's quote here. Ballmer: iPhone has no chance of gaining significant market share.
3. Chrome OS. Because why would you want to run Windows or MacOS apps on a full size laptop?
4. FreeNAS. Nothing explains your inability to run Linux better than putting FreeNAS on your home server.
5. iPad OS. When even the OS itself does not know if it is competing with Windows/MacOS or Android/iOS. 
6. DOS. If you remember running floppy disks on DOS computers, I only have to say ha-ha, you're old!