17) 10.9 Mavericks – An inexplicable update, the only notable new feature being a slower boot time. This was Apple saying, "We can make them do anything" (evilly rubbing hands together).
16) 10.0 Cheetah – OS X's first release after the public beta. The Cheetah codename was an unfortunate attempt at irony.
15) 10.1 Puma – An OS X still not ready for prime time. "Restore the Apple Menu" petitions were rife at this time.
14) 10.7 Lion – The first release in the iOS era. The UI was a mess. No more Rosetta or Save As. Who thought that denim texture was a good idea?
13) 10.2 Jaguar – Well, at least they removed the pinstripes from the dock. Progress is incremental.
12) 10.10 Yosemite – Glad memory's cheap these days. Also, what was with all the spyware? B phoned home even when you opened "About This Mac."
11) 10.11 El Capitan – All the pros and cons of Yosemite but slightly leaner.
10) 10.5 Leopard – Dropped the Classic Environment yet simultaneously introduced new levels of bloat (mostly useless eye candy). The last to support PowerPC.
9) 10.3 Panther – Brushed metal. It was the best of times, it was the worst of times. Brushed metal.
8) System 7 – Brought many new modern features, but also brought something else we've all become familiar with in OS updates – a slower system. Also, the 7.5 - 7.6 releases had stability issues (mostly related to the PowerPC transition).
7) Systems 1-5 – These first Macintosh systems were rudimentary, but they established the user interface in an unprecedentedly human way. Whereas every other OS made you feel like you were at work, Macs made you feel like you were at home.
6) Mac OS 9 – Like 8.6 but with a bunch of new stuff I never use.
5) 10.8 Mountain Lion – Apple apologizes for Lion.
4) Mac OS 8 – Introduced Platinum, which I've never really been a fan of, although the Finder remained very fast. Had to wait till 8.6 to become stable, then it became like a rock.
3) 10.6 Snow Leopard – Apple apologizes for Leopard.
2) 10.4 Tiger – An OS X that's stable, efficient, and stays out of the way. Still usable as an everyday system after nine years, and in the internet era that's a major feat.
1) System 6 – The apex of speed and usability. Its boot times beat any modern system, and the user interface is the very definition of intuitive. System 6 is largely responsible for making your compact Mac feel like an old friend.