It's all subjective I guess.
I really liked 10.2, did not care for 10.3 at all. But I would declare 10.1 "the worst". Does 10.0 really count? I thought that was released in a beta or "public preview" form.
10.0 was a
real release. There was a Public Beta released just before it.
This is the way I see the OS X OS'ses:
10 Public Beta: 'nuff said. It's a beta. Fun to play with, not great to actually work with.
10.0: Was also fun to play around with. Terribly slow on the fastest Mac out there at the time (G4 500 MHz IIRC). No DVD player, hardly any 3rd party apps, only Internet Explorer that worked, etc.
10.1: Starting to become usable. There even was an MS Office for Mac that worked natively on this OS (!)
10.2: First time the OS got hardware GPU (Quartz Extreme) acceleration.... This meant faster window-drawing in the Finder etc., which meant less irritatingly slow. OS X was really getting there. A lot more 3rd party apps were available now.
10.3: First real usable OS X. All apps were there and the OS was "snappy" enough to be used mainstream. Even on older G4's.
10.4: Gr8 OS. Very stable and very reliable.
10.5: Universal Binary. OS X 10.5 on Intel screamed. Really superb OS.
10.6: Intel only. Very, very, stable. Very well supported. This version is probably the most "mature" version of OS X. OS X Server 10.6 was the last "real" version of OS X Server.
10.7: Quite a lot of new features: Mac App Store, Versions, Launchpad, etc. First time Apple is moving OS X towards iOS....
10.8: Moving on... more iOS-like features, more iCloud integration. Seems a stable OS.