Win 2k8 R2 ;p
But seriously, OS does not matter. The straightness of hands does matter
I've mined on CentOS, Windows Server 2008 R2, Windows 7 x64/x86, MacOS 10.6.8, Windows Xp x64/x86, Windows Server 2003 x64/x86
p.s.
MacOS 10.6.8 - poor OpenCL
CentOS - no voltage control(?)
Windows Server 2003 - problems with OpenCL installation
Windows Server 2008 - OpenCL must be installed manually + setx DISPLAY ":0" for headless - takes a lot of HDD space ~30GB
I'd go for USB *nix(BAMT f.e.) if I'd be certain that I could change voltage either in OS or in BIOS.