I've used at least:
Win3.1 (or was it Windows for Workgroups actually?)
Win8 preview for half an hour or so, if that counts
While Win7 is quite probably technically the best Windows ever (being the newest etc.), the stupid sh*t that comes with Aero makes me still prefer WinXP, so I'll nominate XP as the Windows OS I was the happiest with. Nowadays it may be too insecure for heavy using, but ok for gaming.
Win2000 is close second, because even for games it was pretty much the same as WinXP. It even had almost the same compatibility modes for Win9x games. I rate it lower mostly because for some graphics cards it didn't receive quite as new drivers, and its MS support ended much earlier than for XP (even though it was only one year older than XP).
Still, of them all, Win9x, especially Win98SE, have a special place in my heart. Microsoft had to carefully move forwards with Windows (also for PC gaming), yet at the same time not alienating PC gamers which had lots of older DOS games.
Win9x was quite perfect for that purpose, with its good backwards compatibility with DOS games, and at the same time introducing components which made Windows gaming viable at last (which in turn made it possible for home users to later move to better Windows OSes like XP etc.).
Wow, suddenly games could work fine with non-3Dfx 3D accelerators, or with sound cards which were not HW-compatible with Adlib/Soundblaster! And goodbye to CH Flightsticks, welcome much superior USB flightsticks and gamepads from various companies, with their vastly improved accuracy and features (number of buttons, more axis etc.).
I guess online gaming also became much more viable with Win9x. So, all in all I'd call Win9x as the biggest single achievement of Microsoft. It shaped and enhanced especially PC gaming the most.
Post edited April 12, 2012 by timppu