Linux - most versions; current favourite is Linux Mint!
I don't really like Windows although I do accept it does have it's place and have to work on Win 2003 and Win XP throughout the day does allow me to appreciate the OS X experiece when I get back home! I also like that Windows has the majority - it give the stupid people something to hack, crack, write malware for etc.
I personally quite like Vista - it is fairly stable (in the right environment), looks good (for Windows) and there were a number of improvments that I personally appreciated. Main downside for me is that my sister has a bit of trouble with hardware drivers (read; I am left trying to sort out problems for her) - but that's more down to the device companies than MS.
Although Ubuntu is my favorite, I think you should separate Win7 from the other versions of Windows. With Win7, MS is finally getting Windows right and giving us a real competitive OS. I really liked it, but Ubuntu remains my non-Mac favorite because it is so simple, completely open, fully feature- and software-complete, so fast and small, very elegant and FREE!
All of them. The more obscure and less used, the better. They all do some things really well (whether that actually be pragmatic or philosophically) so in my opinion, it can't hurt to experiment with all different sorts.
Sorry to report but Mac OS X is my second fave OS, only second to what I think is Microsoft's best, Windows XP Media Center Edition 2005, sure it's nowhere near as new as Snow Leopard however it does contain some real goodies that in a PC I appreciate. The Media Center application works seamlessly with my tuner and does a good job of PVR'ing the shows I want to watch later.
Plus it's quite speedy especially when you can run it nicely on lowr end hardware however, my PC isn't lower end but I do have one slow machine that loves XP.
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