The original Macintosh up through System 7 was perhaps peak of OS UI design, the simplicity and obviousness is a marvel. The modern Mac experience is no where near that simple, particularly since the Yosemite UI overhaul.
The original iPad had pretty great UI design too, until the iOS 7 redesign. Now I think it's probably the most confusing UI that exists on a modern OS. Nothing is obvious. Multitasking and copy/paste on iPad is befuddling to even the smartest and most technical people I know.
UI design that was perfected 30+ years ago has been completely shelved and lost by designers who are designing for other designers and their portfolios, rather than for users.