It feels a little clichéd to say it, but I do feel that Snow Leopard, outdated now as it is, was some kind of fortuitous confluence of factors that resulted in OS X being as close to perfect as it's ever been.

Even putting aside the iOS-style elements being added, my experience has been that each version of OS X has been slightly worse than the last, and tends to introduce strange little anomalies and instabilities on hardware that was otherwise working just fine. Sometimes these issues are fixed in the next major version, but sometimes they aren't; and even when they are fixed, there are an equal number of new issues introduced.

My perspective is perhaps coloured by having switched to the Mac during the Tiger era, which was about equal with Snow Leopard in terms of stability and 'completeness'. Since then, with the exception of Snow Leopard, it's been downhill. (I realise, of course, that Tiger benefitted from 11 point updates and thus more polish than any version before or since, but the point stands.)