Os X
I have the Panther running on my iMac, mainly because my Garageband projects didn't work any more with newer versions;
on my MacBook I ran Tiger till early 2009 (and still on an external drive),
now 10.5.7 with which I'm very unhappy.
10.5.6 did mess up the connection with my router; that's better now on 10.5.7 but Safari 4 is not working as well as Safari 3 on 10.5.5.
Though I have TimeMachine backups of the previous versions I don't now how to migrate to an earlier system with keeping all the new apps I have installed in the mean time, if so, I certainly would go back to 10.5.5. wit Safari 4 beta.
I do not plan any snowy update and am frankly amazed how everyone updates and then complains about incompatibilities.
Next year I will acquire a new Mac, with 10.6 of course, but if Apple further refuses to provide quadcores and blueray in affordable desktops and laptops it will be a clone or a PC, meaning bye bye Cheetah and welcome, well, whatever runs on Windumbs...