To me it's always been win2k, win 7, win 10, win 98. In tthat order. I've been using win2k rather than windows xp when I need legacy application support, though you might need to jump through some hoops for XP compatibility and drivers.
Though I've been inpressed by how snappy windows 8 feels, especially the boot time which is the fastest of all versions.
8 was the first version that hibernates the kernel on shutdown so it can reload it from snapshot on boot. That’s why all windows versions since 8 boot so fast.
Still Windows 8 boots faster for me. A clean installation is 2 seconds from bios to login (not hibernated, but "fast startup"), and perhaps 5-6 seconds for windows 10 (also with fast startup enabled, which it is on both on a default install).
Though I've been inpressed by how snappy windows 8 feels, especially the boot time which is the fastest of all versions.