What I would do for Snow Leopard on the M class hardware.
It was coherent, (relatively) bug free, and lacked the idiot level iOSification and nagging that is creeping in all over MacOS today.
I haven't had to restart Finder until recently, but now even that has trouble with things like network drives.
I'm positive there are many internals today that are far better than in Snow Leopard, but it's outweighed by user visible problems.
It shouldn't surprise you I think that Android Jelly Bean was the best phone OS ever made as well, and they went completely in the wrong direction after that.
Programs absolutely could have much more controllable auto save before for when it made sense.
Speaking of security it didn't have app sandboxing either.
This is what I mean about iOSification - it's trending towards being a non serious OS. Linux gets more attractive by the day, and it really is the absence of proper support of hardware in the class of the M series that prevents a critical mass of devs jumping ship.