Well, I think I have to change my mind (thanks to Richard's parenthesis), though it is a close run between Gandhi, the man of peace and the man of war. Churchill is obviously the most important person of the century, in spite of the newsreel cliches, and his limitations, and the endlessly tedious British self-congratulation, and the greater sacrifices of the Russians and so on. Coldly appraised, if it wasn't for him, it is pretty obvious that Britain would have fudged around, or at best fought to a temporary draw, and Hitler would have taken over the rest of Europe, and then Britain in time. Once Hitler had Britain, an American war for Europe would have been logistically impossible until the late 1960's (if ever). The war between Stalin and Hitler would have been even more hideous than it was. One can only begin to imagine what a Europe freely run by Hitler, and either winning or losing to Stalin would have been like. The War itself was only a grim warmup to that possibility. And a short delay time would have brought nuclear weapons. And if it wasn't for Churchill's friendship with and influence over Roosevelt, the Americans would have probably fought the Japanese first (assuming the Japanese still went to war which they were heading for anyway), and not second, and Hitler would have again won the battle of Europe. It was all so close.
We would probably be in the midst of, or the ruins of, huge continental atomic wars in the 1960's between the North Americans and the Russian or German conquerors of Europe.
The British crisis in 1940 might possibly have thrown up somebody who could do what Churchill did, but it was Churchill, after all. When you think about how close the appalling alternative last half-century was, in spite of everything, it has to be him. IMHO
Yours, Peter T.