I'm going to set myself up for a karma decimation here, but I think MS already had the best UX of any platform, they just blew it by trying to smear it onto every device in existence.
Windows Phone 7
This was, and still is, a UX that fitted the device perfectly, it is intuitive, efficient and beautiful.
Some 'for instances'
why do we need a 'button'? This is already an artificial construct; if an idea is captured by text then touch the text to access the associated sub-levels. Or headings that extend beyond the screen; so you naturally swipe to see the end of the title and you get to the next page with a new title.
I am probably as upset as anybody for how Windows8/8.1/10 has evolved but for entirely the opposite reasons of most people.