My favorite version of Windows overall is probably XP Professional 64-bit edition, though on my laptop I'm currently running Windows 7 Home Premium. I couldn't find any 64-bit XP drivers for my sound card or GPU, and I didn't want to limit my machine's performance by going with the normal 32-bit version of XP. Not to say that the 32-bit version of XP is bad, as I still run it in a Virtualbox VM for apps with compatibility problems, but there's no point on installing it on a machine with more than 2GB of RAM, and my laptop has 6GB.
I've also used various distros of Linux, two of my favorites being Crunchbang and Linux Mint. Linux is a great OS for day-to-day tasks like writing documents and surfing the web, but the one thing that keeps me from switching to it full time is the fact that it's absolutely terrible for gaming. If you're lucky enough to have a supported graphics card, you can get certain games running through Wine or the native port of Steam, but it's not really worth it since Windows still delivers better gaming performance, and much much better compatibility.