I have Windows ME (Yeah, I know, but it's actually working better than 2000 right now.
) Windows 2000, and OS/2 Warp 4.52.
Windows 2000 works with IE 6, FireFox, Sea Monkey, Office 2000 Pro, SmartSuite ME, and Works 99. (99, or 4.5a, is the best version of works AFAIK. All they did in the later versions was take out the templates that made it so good and downdate the look.)
OS/2 works with SmartSuite ME, FireFox, and a lot of Windows 3.1 software including some games.
While it is obviously the best mainstream OS for PC's, I don't know if there is an equivalent of Encarta Reference suite, Microsoft Home Publishing, Broderund Family Tree Maker, etc.
So, while I hate to repeat an old over used line, "It depends on what you want to do with it". (I hate that line.)
I really don't consider an OS "obsolete" until you can no longer connect to the internet and view most web sites with it. So Warp 4, like Windows 95, is still "up to date". SmartSuite probably does more than you'll ever need in a basic office suite. What about the practical home/personal/family & reference software? I hope there's more of that than I know of.
I don't know much about games. I have maybe $10.00 worth ($0.75-$1.25 at the Salvation Army; $0.25 at the local thrift shop) on my CD shelf, A collection that has been growing steadily for about 5 years. Once or twice a month, I fly around in space for half an hour and get shot down.