I think his biggest achievement was not winning the war--that took much more. His biggest achievement was not losing it--as it easily might have been, in June 1940, when some colleagues urged Britain come to terms with Hitler.
The journalist Charles Krauthammer puts this best:
"The uniqueness of the 20th century lies not in its science but in its politics....It invented the police state and the command economy, mass mobilization and mass propaganda, mechanized murder and routinized terror--a breathtaking catalog of political creativity....Totalitarianism came and went. It has a beginning and an end, 1917 and 1991, a run of 75 years neatly nestled into this century. That is our story.
"And who is the hero of that story? Who slew the dragon? Yes, it was the ordinary man, the taxpayer, the grunt who fought and won the wars. Yes, it was Britain and its allies. Yes, it was the great leaders....But above all, victory required one man without whom the fight would have been lost at the beginning. It required Winston Churchill."
See more on Churchill in Richard's upcoming book “Winston Churchill, Myth and Reality”.