As to "the best OS Microsoft has ever made," it was OS/2 (which they abandoned in favor of the "hugely successful" Windows 3.0 which was clearly inferior on all technical accounts).

That was made by IBM AND Microsoft, but if we go so far Xenix was the best os MS ever made/buy'd ;)

I used Server 2003 R2 as a daily driver on a laptop for a couple of years pre-Windows 7. It was very nice. Most Windows XP software would work fine.

Yes 2003 64-bit was the first official 64-bit x86 windows and is probably the largest advance over 2000, and there really hasn't been anything since that is as large a technical capability addition.

I'm on the fence, but frequently say that 2000 was probably the best. Then I have to qualify that maybe the preview 64-bit XP or full win2003 might be first or in the running because 64-bit is such a major/critical feature to modern computer that if you wanted to sell a modern "Windows for business" release you would have to pick one of the 64-bit versions rather than rolling back to 2k.

I too ran 2k3 (64-bit of course on my athlon64) for a while as a desktop and technically it was fantastic, but eventually ended up switching back to XP with a few hacks to just avoid the general issue that a number of pieces of desktop software refused to run on a "server OS" without buying the server license for $$$$.