I still think the biggest user interface (UI) change from Microsoft was the change from Windows 3.1 to Windows 95. There were so many features and improvements

I still think the biggest user interface (UI) change from Microsoft was the change from Windows 3.1 to Windows 95. There were so many features and improvements e.g. the re-designed Open/Save dialog (not much changed since then). Later versions of Windows have never made the same leap between new versions of Windows. Sure, there are refined or tweaked features (or regressed!), but each new version of Windows felt like an incremental update from the previous version. I think the next leap in UI and visual design from Microsoft was...Windows Phone 8 (2012). It was a refreshing and different approach to mobile design. The visual language 'Metro' worked well on mobile touch screens. But Microsoft took the bad decision to transplant 'Metro' (flat design) to the desktop. The result is a clumsy, jumble of inconsistent screens. It simply doesn't work.