Windows 7 because it works just about flawlessly and it's beautiful. I'm checking out possible Linux successors though (looking closely at Kubuntu) because W7 is 6 years old. It'll need replacing eventually and I won't be moving to W10. I call its UI "Leon Spinx" after the boxer who Mohamed Ali once said was so ugly, it looked like a train went through his face.
Only two things are infinite, the universe and human stupidity, and I'm not sure about the former.
Albert Einstein
Albert Einstein