7 Ultimate x64, pirated edition.
I only have 4GB of ram currently so the x64 is a bit moot, but I'm planning to upgrade to 6GB or 8GB soon.
For those who want to stick with Windows but haven't taken the plunge into 7 (still sticking with XP or Vista), I can say that it's an extremely worthwhile upgrade (if tedious, regarding the format/reinstall process). Just make sure your hardware is within the recommended specs (decent CPU, discrete graphics, etc). I would also suggest having 2GB ram at the bare minimum, and 4GB for a good experience.