XP. I had nLited version using 80MB RAM. I used it for a year with IE7 and no updates, no AV, no firewall. I have never got a single virus and it was lightning fast.
I've used all OS since Windows XP (and Windows 98). No doubt best one ever created is Windows 10 combining speed, security and compatibility. After that, i'd choose 7 and then 8.1.
windows 7 for me never had a single problwm with it! I have had major agrrivations with win 10 allready several times , always with those annoying update error codes!
W2K Pro SP4. Built like a brick (out)house, ease of management, worked flawlessly until the box developed Alzheimers in 2012 and I had to upgrade to W7... Which is nearly as good, but will never be better.
The box actually came with XP loaded, but I formatted and installed the better OS