Windows 7 for me, though I wouldn't mind trying out OS X once it makes it so your can have windows stretch out to every corner of the screen.
Currently running on Windows Vista, but reading what Geoff mentioned to Drew, I'm going to have to look into that, because I'd love to be running on Windows 7. However, if I ever have to purchase a new laptop in the future, I'm going to reach out and buy myself a Mac.
I've been harcore about windows my whole life and mega anti-mac. Recently I've been pretty fed up with Windows sucking ass, so I've switched to Ubuntu. After about a week of running it, I'm primary Ubuntu 11.04 right now. Been working out pretty good, aside form the lack of gaming ability. I still have Windows Vista on my hard drive for running games, but there's patches I haven't tried from some.
Well apparently Windows 8 is coming out sometime soon. I've been hearing a lot about it. Windows 8 will probably upstage Windows 7, somehow.
It's better than XP. :( Though at least the computer I work on now has IE8 instead of IE6! (Except it also takes a whole minute to load the first page because of all the toolbars they've installed on it......) Actually all I've seen from Windows 8 so far points to them trying to pull an Android with it, making it an OS that works for phones, tablets, and computers. And you'll note that Google gave up on that and saved Android for phones/tablets and moved onto their Chrome notebook instead.
My primary OS is Windows 7, it is the first OS I went to from Windows XP I completely skipped Windows Vista as I could never get on with it.
I have a mac and just recently I got Lion for it. On my PC I run Linux though. But I have to admit that when I'm not gaming I'm definitely more of a mac kind of person.
I have Windows Vista, and always did. I don't like Windows XP that much. Dunno about Windows 7, though.