Basically, what is your preferred OS and, more importantly, WHY is that your favorite OS? I know that there are some die-hard *BSD fans here, so this could get interesting...
I'll go first - I've primarily used Windows and several Linux distributions, and my favorite OS is by far Linux, especially Gentoo. Gentoo simply gave me more control over my system than anything else I tried, and made it relatively easy to set my system up exactly the way I wanted it. Second, I absolutely love Portage: it's very powerful, but still easy to use. Third, I prefer software for Linux over software for Windows simply because most software for *nix is distributed as source code, which adds another layer of configurability and optimizability. Finally, unlike Windows XP, Linux almost never crashes on me, and it's usually pretty fast. In fact, the only time I can remember Linux crashing for next to no apparent reason was one time when I was using Fedora... Anyway, those are a few of the basic reasons I prefer Gentoo - customizability/configurability, Portage, speed, and stability.
Now let's hear it from some of you guys... =P
-Ch4r