Win 95 but the oldest I have used was Mac OS 1 but it was not called that.
http://download.info.apple.com/Appl...ates/English-North_American/Macintosh/System/It was called Mac System Software: http://en.wikipedia.org/wiki/History_of_Mac_OS
Windows 3.1 here, too. Lots of disks.
Only needed 4, for company-made disks. After all, Windows 3.1 was just DOS with a GUI shell.