As for performance, I think Microsoft is going to surprise people. And if you're working for an enterprise that hasn't upgraded to Vista because it won't run acceptably on your existing PCs, you'll want to look again at Windows 7.
Paul Thurrott, Supersite for Windows
Back in 1978, a floppy drive was a pricey, leading-edge peripheral--think of it as the Blu-Ray burner of its day. A surprisingly high percentage of Apple II owners had them--most of us were still futzing around with storing programs and data on tape cassettes--and it was Apple DOS 3.1 that made it possible. Despite the version number, this was the first commercially-available disk operating system for the greatest PC of all time; the earlier iterations never reached the market. You might argue that it wasn't, then, an OS upgrade--but it was certainly a major upgrade to the capabilities of the Apple II, which had been on the market for less than a year.
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3. Apparat NewDOS/80 2.0 (1981): Twenty-five years later, it's weird just to think about it, but TRS-DOS--Radio Shack's operating system for its TRS-80 computers--was so crummy that most discerning TRS-80 owners I knew spurned it in favor of NewDOS, a third-party rival sold by a company called Apparat. (Eventually, it was one of multiple TRS-80 alternatives--others included LDOS, DOSPLUS, and VTOS.) Radio Shack probably didn't consider it an upgrade to TRS-DOS, but the rest of us sure did...
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It was the Mac OS--back when its lead on Windows was particularly gigantic--was major additions such as a color interface, built-in multitasking, better stability, and aliases. The best Mac OS upgrade until OS X came along.
5. Microsoft Windows For Workgroups 3.11 (1992): It's not always the upgrades with fancy version numbers and big marketing budgets that mean the most. WfW 3.11 was the most highly-evolved version of Windows 3.x (and yes, I may be making a mistake by not including Windows 3.0 on this list, since it was the first Windows that was actually worth using). 3.11 also made networking a fundamental part of the Windows platform. Perhaps the highest compliment I can pay it: When Windows 95 came along, there were things about WfW, including its File Manager, that lots of us preferred to Win95's way of doing things.
6. Linux 0.99 (1993): In 1991, Linus Torvalds began development of a UNIX-like operating system. That was a big deal. But it was at least as big a deal when, a couple of years later, he decided to release it under the GNU Public License. That jump-started the worldwide developer community that turned Linux into...well, Linux.
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8. Microsoft Windows 2000 (2000):
It was stable, like Windows NT. It was reasonably usable, like Windows 98. It wasn't plagued by spyware, and it felt more businesslike than Windows XP. For awhile there, I thought that Win2K was the single best version of Windows ever released, and while I wouldn't make that case today, it was a vast step forward for anyone who replaced any version of Windows 9.x with it. (Trivia: Six years after its release, it's still the second most-used operating system among PCWorld.com visitors.) 9. Mac OS X 10.0 (2001):
Technically speaking, this wasn't so much an upgrade to Mac OS 9 as a wholly new operating system--based on the NeXTStep OS that Steve Jobs spearheaded during his exile from Apple--that offered backwards compatibility with OS 9. Launched the same year as the iPod, OS X played almost as big a role as Apple's music player in returning the company to its rightful role as the one the rest of the industry rips off. (A remarkable percentage of the "innovations" in Windows Vista are cribbed from OS X.) Most important, for my money, at least, it and its successors are the best desktop operating systems of the modern era.10. Microsoft MS-DOS 2.0 (1983): Subdirectories! Hard-drive support! Backslashes to indicate file structures! This upgrade to Microsoft's operating system, which came along just as PC clones began to dominate the computer industry, introduced lots of stuff that, it's startling to recall, weren't part of DOS from the get-go.
