Which battle of 1942-43 turned the Eastern Front against Germany?

Stalingrad, and the scale is difficult to convey - something approaching two million casualties across the battle, fighting room by room in a ruined city, and a German Sixth Army of over 250,000 encircled by a Soviet counter-offensive that struck the weaker Romanian and Hungarian formations on the flanks rather than the Germans in the centre.

Hitler forbade a breakout and promised resupply by air, which the Luftwaffe could not deliver at anything like the required tonnage. The army starved and froze through the winter and surrendered in February 1943 - about 91,000 prisoners taken, of whom roughly 6,000 ever returned home, most of them years later.

It is the standard turning point and it is worth noting that Kursk in July 1943 was arguably the more decisive military event, since it destroyed the German capacity to mount a strategic offensive at all. Stalingrad's weight is partly symbolic - the city carried Stalin's name, the propaganda on both sides had made it a test of will, and it was the first time a German field army had been destroyed. After it, the direction of the war on that front never reversed.