There are certainly valid and compelling arguments for all the choices. Dunkirk was an error of epic proportions fed by Hitler's delusion that Winston Churchill would be replaced and he could negotiate a settlement with the "Brother Aryans" of England.
Stalingrad was the beginning of the ascendency of the Red Army in the East. It was a massive and irreverisible defeat for the Krauts. It also demonstrated to Stalin that he had a winning team in Zhukov, Konev, and Chuikov.
The BOB, while important, I have problems with as a "turning point." I agree, to an extent, with Stephen Ambrose that the real turning point in the west was D-Day and the German failure to stop the invasion. As Ambrose put it in the preface to his book D-Day, "Prior to D-Day the question was not whether democracy or totalitarianism would prevail, but which form of totalitarianism would win."
My choice is actually the "Big Week" campaign by the 8th Air Force in February, 1944. Beginning with those four missions the demise of the Luftwaffe began in earnest. It has to be remembered that in the late fall/early winter 1943, the 8th was essentially defeated by German fighter defenses losing an average of 15-20% of bombers dispatched on missions into the German heartland. The Luftwaffe owned the skies over the Reich.
The deployment of the P-51 Mustang in squadron and group strength allowed what was possibly the greatest piston fighter* of all time to escort 8th Air Force Bombers all the way to Berlin and beyond. In a series of epic air battles in the Spring of 1944, the 8th Air Force broke the back of the Luftwaffe.
In less than six months, the Luftwaffe went from owning the skies over Germany and Occupied France to Ike's proclamation to his boys going into Normandy, "If you see planes overhead, they will be ours."
The Allied air forces flew 15000 sorties (one flight by one aircraft = one sortie) on June 6. The Germans managed to launch TWO.
WWII was the first war where the side that owned the air owned the battlefield.
In 1933, Hitler had boasted "Give me ten years and you won't recognize your towns!"
It was one promise he was able to keep.
I apologize to the Eastern Front mavens, but an intact Luftwaffe in the summer of 1944 stops the Russkies in their tracks and makes D-Day impossible.
Historian Edward Jablonski documented "Big Week" in two excellent works: Flying Fortress, and AirWar.