Windows 95, the pinacle of UI design. Honorable mentions: Gnome 2 with Clearlooks or Mac OS 9.

On the modern side, SerenityOS is going the right way.

OS X circa 10.6 – 10.9. Aesthetically pleasing, with lots of tonal contrast and touches of color to highlight important items. Clickable elements are clearly clickable, and the UI is reasonably dense, but it never looks overcomplicated or busy.

Windows up until the downfall starting with Windows 8.