As a cross-platform developer, I'd have to say that I like all three, Windows, OSX, and Linux (ubuntu is my preferred distro). Each one has their own little cool thing that makes them what they are. They all do the same thing, pretty much. Why do I use Windows (XP, Vista can go to hell)? Well, most games are developed for the Windows platform, which can be ported to other operating systems, however I feel that I should use the operating system that it was made for.
Viruses aren't a real problem to me. Of course, they're a huge threat for Windows users, however I'm generally careful with how I use my computer.
But getting back to the original point of my post, the best operating system should not really be influenced by its looks, but its performance and how it will get what you want done for you. This is why I would have to say Linux is the best operating system to get things done. I'm not going to get into a huge comparison between all three, but if you really want to get things done quick, Linux is the way to go. However, if you still want that warm fuzzy feeling from your good ol' MS box then stick with it (I know I do) =P