In order of my preference:

Windows 7 Ubuntu 12.04 OS X Windows 8

Windows 7 earns my bread, it works, and most of the time works well for clients and friends. Ubuntu is my goto "Unix/Linux" for OSS development, friends and resolving Windows issues.* OS X, use it part-time at work and friends, couldn't justify the cost of the hardware and some other quibbles. Windows 8 is a UI failure for the power user and admin.