I'm late to the table, having just discovered this thread, but I felt compelled to comment anyway because I am a life-long Churchill fan and student of British history . . . . .
The simple question is not fair to either Churchill or a host of other British prime ministers who are also contenders for the honors. Strong cases can be made for Pitt, Disraeli, Gladstone, Lloyd George, and Thatcher as the "greatest" prime minister, but Churchill is undoubtedly the single most important war-time prime minister in British history, and having rallied not only Britain, the Empire and the Dominions with his defiant rhetoric and belief in the righteousness of western civilization after a series of defeats at the hands of Germans, he was responsible for keeping the Allied cause alive and in the fight until Britain was joined by the Soviets and Americans in the anti-N*zi crusade. He was a great man in every sense, including his occasional flaws of judgment and/or discretion, but a great man nonetheless. We shall not soon see his like again.