Stalingrad was a Meat Grinder. Without it, the D-Day would have been deadly for the Allies. Because the best Divisions from the Reich where fighting in the East. Not to mention that the Landing in Italy would have been in Vain when Hitler would not have pulled most of the Troops stationed there to the Russian Frontline! Also do not forget that not Waterloo was Napoleons Grave, but the Troops he lost in Russia! Hitlers Stupidity not supplying the fast advancing Troops enough was the reason the Germans run out of Ammo and Fuel in Stalingrad. Which was the Reason for the Wehrmachts ultimate Defeat there! Men starving and with Knifes only can't put much of a Resistance to Tanks! The Invasion in Iraq ran in the same Difficulties, that is why they had to stop the Troops for a While to resupply.
Although there were many important battles during WWII, the battle of Stalingrad was probably the most important. The axis suffered a staggering 850,000 casualties and it was the bloodiest battle of WWII. The battle of Stalingrad was the turning point of WWII in Europe. Before Stalingrad, the Germans were winning and after the battle they were losing. The battle of Stalingrad stopped Germany from gaining unlimited quantities of oil in the Caucuses and was the beginning of the end for Nazi Germany.
D-day was more important because it opened up another front on the war. It allowed the us and England to have a larger impact on the war and spread out hitlers forces easing the pressure of the Russian front. It gave the allies a foothold in france and pulled troops away from the german troops fighting on the Russian front.