Macs were at their absolute peak with 10.6 Snow Leopard. Unfortunately Apple badly ruined it with the iOSification update that was 10.7 Lion, with things getting worse and worse from there.
I think the death of Apple (and the birth of modern Apple) was iOS 7. Skeuomorphism, Steve Jobs being freshly deceased (RIP). The UI looked and felt like an undergraduate design project. The minimalism was pretentious and out of touch, and it's only gotten worse in my opinion.