While my first answer would have been Stalingrad (turning the tide in the East was ultimately Hitler's demise), a close second for me would be the evacuation at Dunkirk. Had the Wehrmacht and the Luftwaffe coordinated an offensive in time, the entire BEF could have been knocked out of action and Britain would have suffered a dramatic political and military defeat that could have pushed it towards peace, or at least some sort of armistice. This would give Hitler a free hand in the East while he was still prepping for Barbarossa.
