Some of the worst operating systems I can think of are:
*Windows ME = Even though it ran on top of MS-DOS just like Windows 1.x, 2.x, 3.x, 95, and 98, Microsoft made the boneheaded decision to disable access to real-mode DOS (both the "Command Prompt Only" option at boot and the "Restart in MS-DOS Mode" exit to DOS feature.) Also, the System Restore feature, while a nice addition, was highly vulnerable to compromise and other problems. The removal of real mode DOS led to rumors that WinME was redesigned to run without DOS, like Windows NT/2000/XP/Vista/7/8.
*Windows 8 = They came up with an interface clearly designed with smartphones and tablets in mind, and made a conventional desktop OS out of it, with said interface so difficult to wade through with a mouse. Great move eh?