Not in any order, but:
Mac OS 7 (always in my heart)
OS X Tiger (the first nicest OS X releases in my opinion)
OS X Mountain Lion (much more stable than Lion so far)
Windows 3.1 (you don't get any better software wise, so much stuff made in this era, then it just all died after Windows 95)
MS-DOS (memories of playing Monopoly, using The Print Shop, and other things)
Mac OS 7 (always in my heart)
OS X Tiger (the first nicest OS X releases in my opinion)
OS X Mountain Lion (much more stable than Lion so far)
Windows 3.1 (you don't get any better software wise, so much stuff made in this era, then it just all died after Windows 95)
MS-DOS (memories of playing Monopoly, using The Print Shop, and other things)