well I'm still on WinXP and love it right now.. it does everything that I need and allows me to customize it as I want it. I want to try Vista but don't fell like buying it so I'll probably have it on my next computer (except if Windows7 comes out first, which would even be better).
I also liked Ubuntu/KUbuntu when I tried them.. didn't really explored them enough to say if I'd use it everyday.
And I don't really like MacOsX.. I had to use it for a few months and I don't like how stuff is arranged.. winXP let me chose more stuff and change everything a lot more than MacOS (or thats how it looked for me).
I like Windows XP because it is very familiar but Vista definately has some advantages over it. But the best operating system is probably like Linux for expert computer users that can handle it.