For mine I have to say Windows 2000. It's obsolete now, but I like it because it is just an operating system, and does not have pretensions of being anything else. No fancy clown themes, no desktop gadgets, no compositing effects; it's just a platform for applications, and it just works. Being able to run on really low-end machines also helps!
XP for me (still using it on our 2 older PCs). While 2000 also was a good basic OS, XP added lots of refinements, including plug & play, yet retained most of 2000's efficiencies. I have Win7 on my newer lappy and imho it's just a cleaned-up Vista and should have been released as a Vista service pack (but doing that wouldn't have brought in the big 'bucks' for M$). TS
With the choices limited to NT systems, it would be 2K. It didn't have all the extra garbage that newer NT systems have.
I've used them all. w2k was good. I like XP over it most every time though because of small refinements that made a huge difference. One little one, the network tab on task manager. Those types of things made XP so much easier to work with, to me anyway. Sul.
Eh. Toss up between 7 and 8. If 8 had kept metro and just improved the task manager and the used the new metro explorer it would be 8 hands down.
XP. Stable and able to run on a computer with not many resources. I plan on using it to at least 2014.
I'm only a month and a half into it but I've got to say Windows 7 right now. I have renamed XP my "not responding" and the "hour glass" Operating System. I still have three XP computers.
I'd go with XP SP3 and Win7. As for Win8, I'd need to see the RTM release and use it for a fair deal of time before I decide.
XP once SP2 came out. Before that it wasn't all that great an OS. Surprised I don't see 98SE on this list. That was a good OS too, IMO.
Windows 7 but only because compared to Windows XP I have to troubleshoot so much less. I like both and while I love my W7 I would use XP as an alternate (I think i just spent so much time with XP I got sick of seeing it) if I had to. Windows 8 is a literal crap-hat it's like Microsoft took a hat crapped in said hat and then slapped said hat on windows 7 and shipped it out to market. Vista was a terrible excuse for an OS (Like W8 ) but it is greatly improved however (Windows 7 was what vista was meant to be).
I voted for XP which I had since 2003 (first PC) and just got Win 7 last December. XP was so darn comfortable, just like a pair of worn-in slippers and had it tweaked to my heart's content. Still feeling homesick for XP. It's in storage although it still ran great, except me being afraid the tower/hardware would conk out on me after having had it for so many years. Repair (cost wise) would prob. not be worth it.