I'll second this. I started with the beta of OS X (2000?) after several years with Red Hat, and the period of Snow Leopard through Mountain Lion was the best. Especially Snow Leopard. I recently reinstalled it on an old Mac Mini and a 2006 MacBook Pro and was amazed how fast it was compared to Mountain Lion on a 2011 MBP.

I rue the days I upgraded my iPhone and iPad to ios9. Both are slugs now and they don't sync with Messages on the MBP anymore.