Which are you more farmilar with? What are you going to be doing with the OS? How much do you know about computers and operating systems in general? These are all things you need to take into account.
Mac OS = user friendly, mostly used for video editing, graphic design, etc.
Windows = user friendly, best for gaming, compatible with most software on the market
Linux = higher learning curve than the other two, open source (completely customizable by you), free
That is just a short run down of the OSes meantioned. I am sure that a few other people can add to that list.