Your Operating System of Choice

Windows 7

For gaming: A Win7 toaster.
For work and everything else: Xubuntu.

I find Ubuntu distros to be the easiest to deal with, since it's easier to throw away what I don't need instead of building it up from scratch (though I enjoyed doing so with Gentoo in the past). If I were to find some more free time, I'd play with Arch Linux.

As far as window managers are concerned, xfce and fvwm2 are really high on my list. I can't stand all the integrated shit in KDE or Unity or modern Gnome. Too heavy. :(

I'm currently running Linux Mint and Crunchbang.
Crunchbang is so damn sexy, but the Debian repositories and lack of graphical tools for system stuff(and to a lesser degree the community fetish with sharp fonts) make me spend little time on it.
Mint is really my OS of choice now. It's as newbie friendly as it can be and being compatible with Ubuntu repositories makes it easier to get most things running without running after dozens dependencies and compiling them all, besides being able to take advantage of the huge Ubuntu community resources without actually using Ubuntu.
Gaming was the primary concern holding me back from going full Linux, but I've had few problems with games on WINE so far.

As far as WM/DEs are concerned, I use both MATE(with the stock Marco WM ) and Openbox. MATE is a fork from Gnome2 and thus looks very conservative, which is a big plus for me. I've learned to love Openbox' simplicity through Crunchbang, the ridiculously low resource usage and ease of customization are big pluses aswell. I'm also eager to try Awesome in the near future.

windows 7, fastest windows since 2000... great for gaming and looking at porn at the same time, screw all other OS