FWIW - my six pen'orth
Though I confess - sheepishly - to being a bit of a 'nix fanboy running Ubuntu at home, I've used both family trees (and most versions) of Windows nearly every day for fifteen years or more.
It tits me off to hear Apple and Linux fanboys dismiss Windows as "unusable". Rubbish. The decent versions are stable and capable OSes but the dogs are *real* dogs. And, at default settings, none is secure online.
Windows 2 wasn't much to write home about. But I used 3.11 WorkGroups for many years (on boxes from 386SX to PII) and it was capable and reasonably stable.
I never really got on with 95 - it seemed overly prone to BSOD at critical moments. However, Windows 98SE was fine as long as security wasn't an issue (I reckon it was impossible to truly secure any 9xx for internet use). That said, I used 98SE happily it for years.
I had limited experience of ME but what little I had was *really* bad - Windows ME was a truly awful lash-up.
Concurrently with 98xx, I worked on (and installed across a small business) NT4 machines. Again, very few real problems though plenty of annoyances.
W2K (NT5.1) had a slightly flakey start but by SP4, it was IMO the best of the bunch. Fully patched on anything better than PII/128MB, it was fast and very stable. If hardened (and behind a firewall with Opera or Firefox instead of IE), it was reasonably secure too.
Win XP needed more tweaking, tried to be too-clever-by-half, and introduced a bit too much snooping. But set up right on decent machines, I've used it for years without major problems and quite like it. That said, I still prefer W2K SP4.
My wife's firm has gone the Vista road from XP Pro. Her IT department has been swamped by problems. I've used her Vista laptop (a high spec Dell) and it is a dog. Another colleague has Vista on an older machine and it is even worse. So far, my personal experience of Vista is that it is - to use a tech term - a pile of bloated shit.
IMO the NT line has run its course. Forget backward compatibility and legacy support. I think M$ needs to grasp the nettle and design a workable, stable, fast, lean, and - above all - *secure* multi-user operating system from the ground up. Oh, and supply it in just one or two versions not a mish-mash of flavours as with Vista.
Meanwhile, I'll stick with W2K SP4 and Ubuntu at home and XP Pro SP2 at work.