I had to reply “Vista” although I’m increasingly frustrated with its slow performance, overzealous security, horrendous battery power consumption and constant compatibility issues. Even though I use it, I don’t recommend it to anyone that asks. I’ve purchased an extra hard drive and will be going back to XP as soon as I have a couple extra days to spend on it. Might wait for SP3 first.
I don’t plan to do Linux or Mac, as I would then be just as reliant on virtual machines as I currently am using Vista. At least it would be faster though.