I use WorkBench 3.9 on my Amiga 1200. I will upgrade to 4.0 when it is released in some months though. On my Pc I use Windows 95 (yeah, if you need windows, this is the only version you need. its smallest and works best). However, mostly things I use is Dos-based.
Personnally I use Windows 98 first edition on my main PC, it works fine and it's nearly as small as 95. On my PC used to burn CDs, I use win 95 and when I upgraded it to a k6-2 450 with a MB FIC VA503A, it was pretty hard to reinstall windows 95, I insalled patchs and patchs. It was always crashing and finally I found an update for K6-2 350
Windows XP, I've been wanted to try out linux, but haven't gotten around to burning another knoppix cd (the first time i got an error), or even installing a redhat distro on a test hard drive (I've go no extra, and no room to dual boot). I'm fine with XP anyway.
Win98 on both machines currently running. I use this machine for gaming and internet and the other just to type papers in my room. I'll be upgrading this one to XP Pro soon. My friend seems to think it's pretty solid, so I'll give it a try.
XP Service Pack 1.. After the initial teething problems, it's been fantastic. Obviously you need to get rid of most of the crap from the get-go and disable system restore, after that it runs like a dream (well, apart from some Win95 programs anyways).
I was actually thinking I'd end up setting up a dual boot to 98SE for running my MAME cab, but it's not really been necessary which I've found a bit surprising. If you can't tell, I feared the jump from 9x to NT