It's kinda apples to oranges. I'm not sure there is an ultimate vintage OS. It'd be like saying that the SNES is the ultimate retro console so you shouldn't use the NES. It's apples to oranges.
Some people want to use Windows 95. Windows 98 is a clear improvement over 95, but for the people who want to use 95, they want 95, not 98. Likewise, if you want Windows 3.1 or DOS, then that's the one you want.
That's kinda how retro stuff works in general. Even if a better thing came along later, maybe you want to build a 486 DOS machine because that's what you want. I'm sure some people out there want to use Windows NT or Windows 2000, that's just what they want to use. So it just depends on what kind of machine you like and want to build. What someone wants to use as their retro OS is going to vary person to person. Even with XP, just considering compatibility, there were things that were made for 3.1, or that worked better on earlier versions of windows or on dos. But you also have to consider speed, and the hardware of the era, and the possibility that you might want a period correct machine, and the possibility that drivers may only be available for certain versions of windows, and so on. It's really apples to oranges.