The most significant Apple Operating System, to me, is Mac OS X. This OS bridges the gap between desktop & portable computer OS and the enterprise-grade server OS with its battle-tested Unix core.
Mac OS X gave Apple the most number of switchers from the science and engineering fields. Traditional geeks have found nirvana -- a combination of a beautifully designed graphical user interface with the power of command-line interface of Unix, which is what Linux is trying to achieve, Windows is dreaming to achieve and the old Mac OS failed to achieve.
The Great Tiger Giveaway