Not really "worst OS" but it has one feature, OS'es should NOT have...
C64 GEOS, it has copy protection, sure, you can use original disk to
just boot, and after booting, replace it with copy of it, and it does
work... Untill your original "boot disk" breaks up... Then, you cannot
access any of your works untill you get another original GEOS disk...
Sucks... I think, latest versions didn't have such protection
though...
But didn't Microsoft just introduce much similar system for Windows
XP?
You know, "artificially" prevent user from taking full backup of
(finally) working system, for easier (re-)install later...
Then, about the "good" OS'es, I guess, dr. DOS is best DOS for PC's
(Isn't it called "Freedos" nowadays?)
MS-DOS isn't that bad either; Most likely becouse it wasn't created by
Microsoft in the first place :-)
One rather new OS worth mentioning is Arachne, multitasking graphical
operating system... For unexpanded C64!
Comes with TCP/IP, Web browser and Telnet client