Windows 7 (64-bit) is the sweet spot. Note that newer OS versions like Windows 10 didn't see official support until the 2016 release.
Service Pack 5.0 is the final evolution of the 2012 release cycle. SolidWorks 2012 SP5 Release Notes highlight that this update focuses on maximum stability and compatibility. Solidworks.2012.sp5.0.multilanguage.integrated.iso
The ISO contains all language packs, making it ideal for multinational teams or users who prefer operating in a language other than English. Windows 7 (64-bit) is the sweet spot