Win7
Couldn't get gaming performance up to par on Ubuntu via Wine (unfortunate, since I really like the concept of Linux, and Ubuntu is supposedly the "easiest" to start with). This is annoying because I'm a software engineer and I've decided it just wasn't worth that much more time.
I don't like the Mac UI, and the fact that it's tailored toward non-technicals isn't for me.
I may try Win8 in the future, but it seems like it's built for a tablet (which I don't use or want) and will wait for SP1 to make sure it's stable first.