it all depends
windows - good for gaming
mac - good for image/sound design
linux - good for hosting servers
That's good and all, but you're keeping it too narrow here. Each of these OSes can do each of these tasks just fine; the problem is, people are often narrow minded and biased towards one, and therefore software gets developed for one specific OS. As a result, usually games are set aside for Windows, server stuff for Linux, and art programs for Mac.
Don't believe that each does the others' "jobs" just fine? Here, have some
Source Forge.