Unix
People today don't know what a real OS is. All they've ever used are toys and jokes. VMS, OS/360, OS/400, Multics, Genera...those are real OSes. Unix was created to run a game on a PDP-7. Linux was created as a personal hobby OS. MS-DOS was literally a "Quick and Dirty Operating System." Windows was originally a shell for MS-DOS. Windows NT had some promise, but MS managed to fuck it up by putting the GUI in kernel mode, deleting all of the subsystems besides Win32 and NTVDM, and copying the (lack of) security model from the DOS-based Windows versions. This "worse is better" approach is killing computing.