Snow Leopard is still my favorite. Mountain Lion was ok and Mavericks and Yosemite have been error-filled and support headaches. I am hoping the next version is better tested.
I agree about the testing, but still I think they have managed to keep the technical debt somewhat in control. I really yearn for the day they license Plan9, and port OS X on top of that. Because I believe that would give bigger opportunities, both with respect to hardware, but also with respect to a better foundation for developing more sophisticated features.
[edit: I should say that OpenStep was my favorite release and somethings are still not in OS X which is a pity - as to Plan 9 - I do have dreams of OpenStep on top of Plan 9]