For your benefit, I regularly use Windows, OpenBSD, Linux, MacOS X, and Solaris.

If I was forced to choose only one for a desktop OS, I'd probably go with MacOS X (not that I don't have a few problems with it here and there).

I use Windows because I've got a PC that my wife uses and she's a sloooooow computer learner. Trying to teach her to use a new OS would be seriously painful. (She still barely understands how to work the TiVo despite using nearly every day for two years.) Once I get a new desktop workstation, I hope to retire that machine to her and she can Windows it up all she wants. I also use Windows for a few particular applications -- games and XNews. I need to find a news client I like under some other OS.