I am going to take a diplomatic approach to your question as at current state of computing technology it is not a valid question to ask which is greatest OS.
Windows has the monoploy of having most of the application support. Then again that makes it the same reason why it most vulnerable as attackers target it most. Microsoft handles these quite well.
Macs are good, but again they are similar to UNIX systems, makes them no different from Linux OSs. The advantage is just the added support and bug fixes they have to keep it stable. Also the added application support.
Coming to linux now, my personal opinion is that Linux(UBUNTU for one) are the greatest OSs. My choice is based on fact that they have less levels of abstraction. This allows a user do everything they want with the internal architechture of the system.
Linux is also good for normal users as the GUI systems in them have improved so much over last decade. Support is the greatest ever as its community. It just takes you to llok for right places to get your questions answered. One can always customize they way it worls for them and increase there productivity.
And since linux is free and opensource, no matter what you do in it, there always minimalist chance of facing a lawsuit :D