For people interested in playing classic PC titles, selecting the right Windows version is very important. Each new version of Windows omits some compatibility with older programs and increases support for newer ones. It's fairly easy to find a cheap, working Windows XP machine and XP can run most Windows 95 and 98 games perfectly, but what if you're trying to run Windows 3.1 software or DOS games? You might get lucky with a few programs, but most won't work. However, if you're using an older version like Windows 98, you'll be able to run nearly all of these, but if you also wanted to play games from the mid 2000s, chances are they'll refuse to load.
The truth is, there really is no best version of Windows for Retro Gaming, but there might be one that's perfect for your needs. The one I find myself using the most is Windows ME Millennium Edition. It certainly has its problems, but aside from one BSOD, I've never had any major issues with it. Except for a few Windows 2000/XP games such as Doom 3, everything I want it to run works perfectly. DOS support is limited since Microsoft tried their hardest to hide their command line OS as much as possible, but for my Retro Gaming needs, it's been wonderful. The best part is that it's very easy to configure like Windows XP and detected nearly all of my Hardware immediately. I was originally going to use 98SE, but I had too many compatibility issues with my PC so I switch to ME.
Which version of Windows do you prefer for running classic PC software and what era of games are you trying to run on it?