I recall upgrading from Leopard to Snow Leopard on my 2008 polycarbonate MacBook. I got back 30 GB of disk space, and it absolutely flew. I straight up don't recall seeing a beachball while using Snow Leopard. Apple called it a 'purely bug-fix release', and free of bugs it was.

In my opinion, OS X has been (mostly) downhill after that. The iOS-ification began with Lion, which changed the juicy blue scrollbars to a boring grey, got rid of 'Save-As'—an extremely controversial addition, as I recall it—and added then-newfangled things like iCloud.