Snow Leopard was (and still is) my favourite OS.
1. Reliability - In my experience, I've never had any system-wide glitches and/or bugs on Snow Leopard, and everything just worked. No screwing around with memory leaks and Bluetooth issues...
2. Speed - Snow Leopard is fast! I feel that it is the last OS X/macOS release that can be comfortably used on a mechanical drive. The best Snow Leopard experience you can get is with either a Westmere or Sandy Bridge machine. Everything feels that slight bit snappier compared to a Penryn/Core 2 based machine.
3. Customisation - Snow Leopard is the last release that could truly be tinkered with. Theme-ing was common, with apps like ThemePark, LiteIcon and Candybar. Had a vibrant community too over on MacThemes.net. My personal favourite themes are Aqua Inspirat and SnowTunes.
I find Lion to be an alright OS, had quite a lot of small bugs at release compared to Snow Leopard, and yes the iOS features were half-baked to start off with, but the UI refresh was nice. Mountain Lion is where all of the Lion bugs were fixed, and the iOS features were fixed. Honestly I liked the Linen look, sure it was a huge departure from the "simpler" look of Leopard/Snow Leopard but at least it's better than this monstrosity:
Although I am loving Ventura on my rMBP, I do feel that less care is being taken to iron out bugs. For example, 13.3 introduced a bug where almost all icons in System Settings disappear. Not a dealbreaker obviously, but feels like you're running a beta, rather than a public release that has been out for 6 months.