This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
There is no possible answer to this. You should use the OS you're most comfortable and productive with. Or the one that your company let's you use you in the office. Windows, OS X, Linux and the various other Unix-ish OSes are all fine to get your job done.
The desktops and laptops the company gives us are Windows XP. We run some flavor of Linux in VM's on them (I run Ubuntu, my co-workers have settled on Fedora) because it's easier to manage our RedHat Linux web servers from some variety of Linux. Our internal servers are Solaris or Windows Server. We have been dabbling in mobile development, so each of us has a MacBook Pro and an iPad. Two of us have Android phones and tablets. So the answer to your question is "all of them".