I think you could say that Germany lost the war in the 1930's The war fought from '39-'42 was a very diffrent one from '42-'45. Tatics changed alot and for the first time, on a large scale, huge amounts of long range heavy bombers were attacking the heart land of Germany and thus their abailty to wage war. The Luftwaffe, much like the Red Airforce, was desinged as a tatical force to support the army. While this worked very well from '39-'41 after that stage of the war airpower shifted to the strategic foot. Germany could not do this in the early part of the war and had no answer to it when the ailles started bombing. The brass tax is that Germany did not develop an heavy strategic bombers during the '30's and they paid a heavy price for it durning the war; they were unable to bomb Britain into submission and the Russians were able to move their heavy indrusty into Siberia well out of the very limted range of German bombers. That said hindsight is 20/20.
This point though can be expanded some what by Hitler's capping of wepons research in 1941. At that point it looked like Russia was about to crack,and the USA had not joined the fight yet. Why would German need to develop new wepons when she was victorious with the current stock? Again this is hindsight. That said it stands to reason that when it was fully realized that there was to be no quick victory in Russia research was started again. It was too little too late though, too much time had been lost and too many new wepons were rushed into production, the Panther tank, to name but one. Was this decree by Hitler in 1941 what cost Germany the war? Maybe, maybe not, but it is something to consider.
In terms of battles, it is impossible to say that one battle cost Germany the war. It was a war of attrition. No one defeat could do it. It had to be a series of defeats to grind down the foe! Germany could have survived any one of the battles mentioned above and still one the war. Indeed, Germany could have still won back the strategic advantage in Russia after Staliangrad had Hitler used his army correctly. That is to say if had retreted and kept his army mobile instead of having them did in and issue orders not to allow them to retreat. There is also the question of German ailles at Stalingrad. The Romanian troops were not as well trained or as equiped as the Germans, yet they were fighting on the front; plugging holes if you will as so many German divisions were tied up in N. Africa. I is a fact that the Romanain troops were the first to crack under the Sovite onslaught and this allowed the 5th Army to become surrounded. If it had been an all German army would the Russians ahave gotten through? I think maybe yes, but it may have been that their advance would have been slow enough to let a larger part of the army escape.
This leads me to my next point. Germany's failure to take Britian out of the war. The failure of Op. Sealion allowed Britian to carry on the fight, and alos to carry the fight to the Axis in the Balklands and N. Africa. As has been said by many people this tied up German troops and also held up the Russian invasion. Of course it also ment Britian was a base for US and British bombers to attack Germany which did huge damage to the German war machine. Britain did just enough to distract Germany while Russia recoverd from the inital shock of the invasion. Then the combined defeats of El Alamien and Stalingrad put Germany on the back foot.
I do belive though that it would have been possible for Germany to recover from these defeats. Total victory was, perhaps, now out of the question, but an agreeable peace might have been gained; remember in 1942 the German army was still the best trained and eqqupied in the world. However this is mooted some what by the USA joining the war. This was one of the huge blows to Germany.In 1942-43 It still may have been possible to for Germany to issue enough defeats on the allies to get them to agree to a "netural" peace but only if Hitler lay dead or in jail. As it was that did not happen and with combnation of the the defeats of '42 and the entering of combat by the USA in that year things look very grim for Germany. This coupled with the general lack mobilty of the German army and Hitler not allowing them to retreat sealed Germany's doom.