Short bio: Computer Scientist, FOSS supporter (read more)
Tux Machines (TM)-specific
Herewith, my idiosyncratic, extremely subjective list of the top ten, in chronological order. (Note that rating upgrades isn't quite the same exercise as rating operating systems, period--for one thing, it sort of rules out 1.0 versions, although I include one anyhow.) I'm pretty sure this isn't exactly the same as your list...
1. Apple DOS 3.1 (1978): 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.
2. Apparat NewDOS/80 2.0 (1981):