Not to get into an OS Holy War, but the greatest OS in the history of mankind was 4.3 BSD UnixI think you make a good point, although I'd have a hard time narrowing down to a particular Unix version. I think that BSD Unix was very instrumental in making much of the Internet happen, so it deserves a special place in history.
I also think, in terms of the change it created, for Windows users anyway, that Windows NT was a historical OS. I was doing software development for a Windows 16-bit product at the time that came out, and I remember being floored at how it was impossible to destabilize the OS - no matter what my app did.
And finally, I would say Apple's OS-X should get a special place in history. OS-X brought a new, stable platform to Macs (much like Windows-NT did for PCs). It took a lot of guts for Apple to abandon the old OS, and move to this entirely new thing, but what a great decision that turned out to be.