Mine is a bit complex:
WB1.3: 1989-1991,late 1995-early 1996: so 2 1/2 years total as main OS
Win95: late 1996, early 1998: 1 1/2 years total
WinNT 3.52: early 1997: 3 months (then went back to Win95 - no software/drivers for NT!)
WinNT 4.0: early 1998: 1 month (once again, driver problems - went back to Win95)
Win98: 2002: 5 weeks (buggy crap - had to reinstall 3 times!)
BeOS: 2002: 3 weeks (great, but no software!)
WB3.1: late 2002-present: 3 years
WB3.9: 2003: 3 months (went back to 3.1)
So for me the winner is ... Nothing! Notice that I used no OS between 1991 and 1995 and between mid 1998 and 2002 - for a total of 7 1/2 years. Other than that, WB3.1 is the winner at 3 years. (not counting the crap I had to use at work)
