Let the flamewar begin!
I'll start. I use Mac OS X, the greatest OS on the planet! Its sleek, elegant design, excellent security, ease of use, and powerful tools make it infinitely better than all the competition!
OS X is made by Apple, a company world-renowned for the beauty of their products. OS X is simply beautiful, as is the hardware it runs on.
Windows security is notoriously crappy. A Windows fanboy will tell you that it's the biggest target, so people target it the most. Wrong answer. OS X's UNIX base makes it practically impossible to hack into. There has never been a virus written for a Mac, even when a $50,000 reward was offered for whoever could write one. (offered by Macworld)
OS X is renowned for its UI. From the dock to Expose (just pretend there's an accent on that. I don't know how to add it in Windows, which is what I'm writing this on), the Finder to Spotlight, Macs function as an extension of your own mind, allowing you to easily perform complicated tasks. Windows is still far behind in its UI, and Ubuntu, the supposedly easy to use Linux, falls behind even Windows.
Apple's iLife makes it easy to create beautiful movies, images, and manage music. And Apple's iWork allows you to create exactly the same things you can with MS Office, but without worrying about the program crashing or trying to find the function you need. Windows comes with what, Paint? Movie Maker? If you think these apps are good, you need to try iPhoto and iMovie.
OS X: Beautiful, elegant, powerful. It enables you to do things that are impossible on any other platform, all without worrying about viruses, crashes, or, really, anything. It is the only choice for anyone who wants to get the most out of their computer.
None.