Windows 95, that brings back memories - bad ones. I was still at university, and working on my diploma paper, we needed to use SPSS, a statistic analysis software, and it was buggy like hell and not well-adapted to the new OS. So much time wasted when needing to manually re-enter complex data sets after some technical glitch, and so often erratic results fed back by the software. Mentioning Windows 95 and especially SPSS 6.0 still gives me the shivers.
Heck, during the last year an university I still took courses in MS-DOS.
It was also the time when I migrated my gaming from Amiga (!) and Playstation 1 to PC. Not before Windows 98 I fully arrived there.
Win 98, XP and especially Win 7 for me remain to be the best versions of Windows ever.
If game compatability would be given, Linux Mint would smoke Windows 7 in a pipe, however.