Well, all of them have their purpose. I mean, Windows is all-around best for maximum compatibility, but regardless of whether it's XP or Vista, they're both pretty bugged. Vista has cleaned up a lot since it came out, so that's an improvement. Unix is by far best for simplicity, Linux is great for being a secure server, technical things, hacking, etc. Macs are relatively expensive, as are all of Apple's products, but they do a good job of having maximum interactivity between their products. They all have their ups and downs, and you have to buy the OS to suit your needs. It's as simple as that. As far as all the qualities necessary for the perfect OS are concerned, no OS has them all. There is no best one.
Null vote. I'm sure most people just voted for the OS they're using even though most of them have never used more than one or two, anyway.