RISC OS
Co-operative though it was, it was at the time leap years ahead of Windows 3.1 and then Windows 95 came along stealing the task bar concept (in fact the 'Icon Bar' pre-dated Windows task bar and Mac's dock. In general UI experience, RISC OS was speedy compared to clunky Win 95 and its pre-emptive multitasking (and whilst were on it, why is it that Windows even today can still take down the entire computer if a pre-emptive multitasking process locks up?).
C64 OS... not even remotely in the same ball park. Didn't even have a windowed interface. C64 was equivalent to the BBC Micro in terms of OS, with barely even any form of multitasking at all, and even then MS DOS was way more advanced, and RISC OS was way beyond that. RISC OS was near equivalent of Windows 95, just without the pre-emptive multitasking and the hideous registry system.
It's long in the tooth now, but for embedded systems it would be pretty good. Pre-emptive Multitasking just bloats Symbian, but otherwise the two have similar footprints. Cut out the desktop and you could build an embedded RISC OS that would be pretty rock solid.
Best thing about RISC OS was installation though.
Forget registries and installers. Just click on the app, drag to folder, job done.
Oh, and let's not forget the super speed of loading the OS thanks to putting it into ROM, and the inherent security this provides. Again, ideal for embedded systems, and if there's no registry/internal database concept (which Symbian does have), then if anything goes wrong, just power cycle and the device is back to factory default. Dodgy app? Just click, delete, gone.