We are all a product of our upbringing but it is interesting to see how the US members place a big emphasis on Pearl Harbour whilst us Europeans (I use the term in a geographical sense) seem to think the defeats in Russia more significant. For my tuppence worth, the iconic struggle in Russia, particularly at Stalingrad as it bore Uncle Joe's name and swollowed up so many German soldiers was the defining point. Also, had Russia been conquered the vast oil reserves in Russia would have made up for any lost in the Middle East. The USA was already in the war, albeit in supplying arms and equipment and would have joined at some point I am sure. El Alamein was important to us Brits because it was the first time we had beaten the Germans in a picthed battle and it was the only front in the Western theatres where British land troops were engaged with the enemy. But as the great WC said himself, "This is not the end, nor is the beginning of the end, but it is, perhaps, the end of the beginnig".
So, to surmise, above all else, the German defeat at Stalingrad.