Ah! You have found the "hidden" link on my preference in operating systems! Good job. You are either very observant or extremely bored.
I find I am most productive in GNU/Linux ("FREEEDOMMMMM!!!"). I run several Ubuntu machines at home and in lab. I used to keep Windows around for playing games on occasion, but recently games have become "supported" enough on GNU/Linux, that I don't need to anymore. Since my wife has used Macs for as long as she can remember, I have gotten rather familiar with OS X.
Several years ago I realized that each OS has its place. Windows is for compatibility (though that is becoming less-so). OS X looks good and works relatively well (or so people claim). GNU/Linux OS's are powerful, customizable and (perhaps most-importantly) Free. In my opinion this makes GNU/Linux come out on top by a wide margin.
I find that the issue of freedom is an important one when it comes to the question of which operating system to use—and more generally which software to use. I've had several conversations with friends and family on the topic and I'd love to have more. I'm still not 100% sure which side of the "Free Software vs. Open Source" debate I am, though, I realize I'm tending more and more toward Free Software.
Thanks for reading!