MPE/ix -- Hands down. Absolutely worst OS ever. Although, security for it is pretty solid, who the h*ll would want to break into an MPE box?

I hate the 6 line file equations.
I hate the account/group/session crap.
I hate having to logging on with 2 passwords.
I hate the lack of any logical organization for files -- they all seem to just be thrown into one place.

Did I mention it sucks? The only redeming quality about MPE is that it has a POSIX shell!