The icons for Leopard-era programs were outstanding. Look at that dark indigo ink jar for Pages, or that wormhole graphic for Time Machine. The comforting smooth grey gradient of window title bars, contrasted with the large, globular traffic light buttons. A typeface that worked well with the lower-resolution displays of the time, and unique icons for everything at every single size. Apple actually had a massive human interface guidelines document, which was promptly binned with Yosemite.
On Windows, that dark blue Start orb and the cool dark task bar, signalling a whole new OS experience. The new Welcome Centre. Freshly rewritten programs and new ones like Windows Media Player and Windows Photo Viewer, and the absolute beauty that was the Windows Media Centre. Flip 3D, customising the glass window borders, and the huge, high-resolution 512 × 512 icons of the high-quality, no-ads games shipped with Windows Vista and 7, which still stand up to this day.
Happy to die on this hill defending this opinion.