Now that's an interesting question. Which version are you most enamored with? Let's start by translating the version numbers into names. Windows 5.1 (client version) is Windows XP. Windows 6 client version is Windows Vista. Windows 7 ... well it doesn't exist yet, but it has been in the news this week.
Earlier this week, Bill Gates announced that Windows 7, code name of "Blackcomb" (named for a ski slope at Whistler, British Columbia), would be available sometime in 2009. Now unless you're in love with Vista, that's exciting. (And if you've installed Vista and like it, you may have been annoyed that there could be a new OS by next year.) However, by the next day, Microsoft announced that Windows 7 would not see the light of day prior to 2010.
A lot of people have resisted using Vista, myself included. If you're running a laptop, performance could be better. If your laptop runs with 64-bit hardware, Vista can be really nice. Now my saying I don't like Vista is almost blasphemous, since as a former Microsoft employee I had ample opportunity to "drink the Kool-Aid." However, I need an operating system that I can use as a host system to run virtual machines on (which I need to do when I'm writing technical books), and XP has less system requirements than Vista. Not to say I haven't looked at it, I'm just - like I did with Windows 95, being slow to jump on the bandwagon. Who knows, I may end up waiting for Windows 98 .... er Windows 7.
There are some nice features in Windows Vista. But it has hefty hardware requirements, although its certainly not the first new version of software that was guilty of that. Opinions? Thoughts? Are you staying with XP? Upgrading to Vista? Going to Linux or elsewhere?