Windows. There are still lots of software companies that don't care about Macs and their OS(OSX I believe). I have a couple of friends that go through hell to run games on their Macs. Also, it's not that you don't get a lot for your money but you're getting different things for your money when it comes to PC versus Mac. I'll admit that Macs are good at certain things but overall I'd say PC is and always will be king.
And that is not true what you said about "no virus can affect* Mac". Yes, they can. The fact is that not many exploiters bother with Apple because of their small market share, around 12% last time I looked. The "Flashback" and "Fakeflash" trojans are hitting Apple users hard, all over the world, right now.
I think all Os are good. But people are generally go for those which is user friendly. Windows is prevalent in business environments, and has differing architectures based upon which flavor is installed (there are vast differences between 3.11, 95, 2000, XP, Vista, and 7) and the variations would best be researched on a product information site. Because of the popularity of Windows, malware and viruses are numerous, and an antivirus program is often the bare minimum that a user must use to protect their computing resources and data.