Snow Leopard was the peak, in my experience; like you, I felt like each new version of OS X through 10.6.8 came as an improvement. Since then, I greet each new release with a sigh - "okay, let's find out what previously straightforward process they've decided to complicate with a bunch of fancy animations this time".
I think people mythologize Snow Leopard. Personally I had multiple very annoying issues[1] in SL that were never resolved until I upgraded to Lion. While I wont say that OS X release after SL have been bug free or had fewer bugs than SL it has always been a trade off - some things got better and others got worse.
The only constant is iTunes, it seems to get progressively worse with every release :(
It's not that Snow Leopard had fewer bugs than its successors, but that it was the last update which clearly had fewer bugs than its predecessors. Since then, each new version has changed things around without seeming to make life better overall.
What's more, as the iOS tail has come to wag the Mac OS dog, Apple's concept of an "improvement" has increasingly diverged from mine. Most of the things they've added since Snow Leopard have ranged from useless to annoying as far as I'm concerned. I don't want an app store, I don't understand workspaces, the new fullscreen mode makes my second monitor worthless. Every new version seems to have new hotkeys and swipe gestures that do strange and incomprehensible things when I trigger them by accident, which I would disable if I had any idea what they were called or where to look for their settings.
I don't think this is just an age thing, either, or a romanticism about the past; my family got our first Macintosh in 1985, and I have used every single version of Mac OS since System 0.9. There have definitely been better and worse eras. I have nostalgic feelings about System 5, and to a lesser extent about Mac OS 8. I don't so much feel nostalgic about 10.6.8 as I have simply felt annoyed by each update since.
