For me OS X topped at 10.6 (Snow Leopard)
It is the most stable OS I ever used, and I can't think of anything useful (to me) added since.
Unfortunately it is not supported for a few years now (security?)
- iTunes: has not been opened for months
- iPhoto: why can't I delete it?
- iMovie: uses it now and then. But I was happier when it was simpler, as what I want it for is simple stuff.