I would say Churchill's biggest virtue by far was recognising the size of the Nazi threat early on. But his military leadership was a litany of terrible incompetence (as early as Gallipoli in 1915): e.g. France & Norway in 1940, Greece, Singapore, Dieppe....
I believe the conventional wisdom these days is that Hitler blew two huge opportunities early on: Dunkirk; and the Battle of Britain through the tactics he imposed on the Luftwaffe, such as targeting civilians instead of RAF bases.