OS/2's WPS is still the best UI I've used. This is an entirely subjective observation, and I'm not like most people. Windows (pick any edition) is the worst UI I've ever used. Mac UI seems to be the most user friendly for the average person. i.e., not someone who wants to dig under the UI. *nix has always had the best command line.

It would have been interesting to see what Atari or Commodore would have evolved into in a world without Microsoft's anticompetitive monopoly.