Earlier quoted context omitted.
OS X 10.6 (Snow Leopard) is no longer supported, unfortunately.
The last good MacOS
Consequently, 10.9/Mavericks is my actual favorite version, and what I'm typing on right now.
When I decided I was going to downgrade my main machines earlier this year, I did a lot of comparisons of Snow Leopard, Mountain Lion, and Mavericks. I could not see a compelling reason to pick Snow Leopard or Mountain Lion.
Mavericks performs well and never seems to crash. When I loaded all three up in VMs and starved them of resources, Mavericks remained the most responsive, I suspect because it supports memory compression. (Interestingly, Snow Leopard lists 1 GB of memory as the minimum required, whereas Mavericks lists 2 GB, but Mavericks still seemingly did better with 1 GB than Snow Leopard.)
Design-wise, while I prefer Snow Leopard's higher contrast in some places, it has a tendency to look either garish or plain in others. 10.8/9 look more refined, without going all flat. And while 10.8/9 have a lot of stupid defaults, I have a setup script I've been using for years which fixes most of them: https://github.com/Wowfunhappy/Mac-OS-X-Wowfunhappy-Setup-Sc...
The one big advantage of Snow Leopard would have been compatibility with PowerPC apps via Rosetta. But, there are only a handful of PowerPC apps I'd actually want to use, and a lot of Intel apps that support 10.9 but not 10.6.