Originally Posted by Plan9
Favourite operating system every is such an impossible question to answer. I'll always have a fondness for my old BBC Micro and Amstrad CPC 464. And while not an OS in the modern sense of the term, Amstrad BASIC was (in my opinion) much more sophisticated than Microsoft's variant that was shipping on many an American computer (eg Commodore 64).
Then there was the rivalries between the early GUI OSs - to which I never understood how Windows became so fondly talked about as my recollection of it was one of the worst of it's time. GEM (Atari) and AmigaOS (Amiga) were my personal favourites.
A few years later and BeOS arrives on the scene. BeOS was an epic operating system and one that really put Apple to shame as at the time Mac OS was failing, badly
. OS 9 had become really long in in the tooth (Apple had been trying to phase it out since the OS 7 days IIRC but still hadn't been able to produce anything equivalent at that point). In fact BeOS was so good that there was talking about Apple buying the OS for it's successor before they went with Next for NextStep + Steve Jobs.
However Apple weren't the only ones looking towards the future, Microsoft were finally working to replace the horrid line of DOS shells that were Windows 9x. While most people favour XP over Windows 2000, Win2k really started the NT reign on consumer PCs and really proved to the world that Microsoft could -if they put their mind to it- actually produce a stable OS.
Just a short few year prior to this desktop revolution, Bell Labs were working on changing the server landscape with their success to UNIX (and my namesake). Sadly Plan9 never really took off beyond basic academic attention, but even so, I believe it deserves a mention because of it's pioneering design; a design has been borrowed a lot by Unix-like OSs since. Essentially, everything on Plan9 was a file system object. So -for example- the GUI was mapped in memory via the file system - and to the point where you could control windows by changing files on the file system (and obviously we're talking about RAM disk files here rather than data stored on mechanical drives).
Fast forward again, and we're at the present day. OS X gets stronger and stronger with each new release, Win7 has shaken off a lot of the stigma with Microsoft's latest paradigm of Windows and Linux has become more than just a hobbyist OS (and my everyday OS).
So to pick one operating system out of all of that (and the countless other excellent OS's I hadn't mentioned: FreeBSD, Solaris, Minix, Haiku, RISC OS, etc) is an impossible question to answer.