Comment by bcoates

For me it's windows 7, if nothing else for being the first and last major Windows where Universal Search worked well.

The Windows 8.x line gets some credit for having the strongest pen interface integration, which regressed significantly in the 10 line, but the overall shell in Windows 8 was rough, and a lot of features were broken in the rushed out and mostly failed attempt to Appify windows and redesign much of the UI at the same time.