My favourites are Tiger, Snow Leopard and Mavericks. And I am more pleased with Mavericks than Snow Leopard which I am more pleased with than Tiger.

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.