I've only been here since 10.3, but my favourite Mac OS X is already 10.6. I'm sure the relevant debates will rage forever and I don't want to rake them over, but I've almost no use for and absolutely no interest in most of the headline features since 10.3.
To my mind, it's only in 10.6 that Spotlight actually becomes slightly helpful (thanks to being able to set the Finder to search the current folder only by default) and I've no interest in the Dashboard whatsoever, so 10.4 wasn't that helpful for me. 10.5's Quick Look and Time Machine are really great features that I'm a big fan of, but I'm with the "translucent menubar is a capricious menace" group, and the 3d dock is another thing that reeks of change for the sake of it. Though it doesn't actually bother me.
10.6 finally makes window minimisation work the way it should, with minimised windows showing up in Exposé and not slowly cramping your entire dock if you wish them not to. It'd still be nice to be able to specify that utility windows should appear in Exposé rather than vanishing into nothing, especially when some idiotic software presents a modal yes/no prompt that then vanishes every time I try to reveal all windows.
I'm not sure about the new Exposé layout yet though. I understand the design principle: giving windows a more uniform amount of display real estate in Exposé mode is smart because smaller windows tend to have important details that are smaller. It does feel like a lot of wasted screen space though, especially when two, tall windows are placed top and bottom rather than left and right.
Oh, and the best bonus: Cheetah 3d renders take about 25% less time in Snow Leopard than in Leopard. I'm assuming some smarter stuff has gone into multicore task scheduling while they were down there working in Grand Central and OpenCL.
I've also no idea when they introduced it, but I definitely wouldn't go back to a time when I had to pay just for QuickTime to be willing to go fullscreen. I wouldn't mind trimming a bunch of the useless bits from iTunes, though. I'm still having to drop to the console to remove the 'genres' column (really, does anybody actually want that? If you like multiple genres you probably don't like pigeonholing, if you like just one then why waste the space?) and iTunes store links.
Xcode keeps getting better too. And updates to that tend to be OS tied.
EDIT: non-Mac, I vote RISC OS.