I've long believed that Windows 98 is the pinnacle of desktop usability on many fronts, 2000 was the best NT in my view. The performance was spectacular, and it was rarely unclear how to accomplish something (unless it hadn't been thought of before the release of W98).

Added: I'd be interested in putting together a Wayland desktop environment around the strengths of Windows 98's interface designs (along with some modern discoveries about human interface, and at least whole number resolution scaling). I feel like there should be at least one well-maintained toolkit which doesn't attempt to support full CSS styling on widgets.