Some of Microsoft's best things have been their improvements of UI over the years.
Metro was probably Microsoft's biggest influence in the days where it was written off as a failure in the Windows 8 era. It put the phrase "design language" and "skeuomorphic" into the vocabulary of a lot of average users, it was a positive influence for both Apple and Google, as Google has emphasized their Material design language, and Apple flattened their UI, eliminated fake glass icons.
The Ribbon has been a complete game changer. Trying to explain to a novice user how to find advanced features in Office programs was a painful experience. Now, the ribbon has put every feature in a logical easy to find place. Even built into Windows Explorer since Win8, the Ribbon makes it easier to do things like show hidden files, show file extensions, etc.