Blue95: a desktop for your childhood home's computer room

This still remains the absolute pinnacle of cohesive desktop environment design in my books.

I think the desktop operating systems of that era were at a sweet spot. They were technically advanced enough to render good-looking, crisp color user interfaces. However, most people were still novices at using computers, so OS designers consciously designed their operating systems to be as clear as possible. Applications tended to be written for each individual platform and to follow its UI guidelines.

Windows 95, NT, System 7 and System 8, BeOS, and NextSTEP all had really clear UX. You always knew where to drag a window, what was clickable, where to find settings, etc.