In my opinion, the versions of Mac OS with the Platinum theme (8, 8.5, 9) have aged quite gracefully. It's clearly not modern, but it also doesn't feel particularly old or kludgy or anything, and it's quite clean relative to modern desktops.

Same as Windows 3.1, and Windows 95, up to 2000. After some point computers began to be optimized for a non-technical person and here we go... Ads, auto-updates, pop-ups, bright colors, all this fucking desktop circus.