>I knew I wanted an operating system from before Apple abandoned the Aqua design language.
I suppose it depends on your definition, but that likely does mean Mavericks is the latest available. For my money though, El Capitan (10.11 to Mavericks' 10.9) was the local maxima (speed, stability, capability). I've no inkling what issues using that would entail—I had no idea that Mountain Lion had "a more capable version of QuickTime"—but my immediate response to this was wondering why not El Capitan.