But it was the allies invasion of italy that made hitler transfer tank divisions from kursk to italy against his generals wishes. so really the allies entering the european theatre was half the reason why the germans lost.
I would say that it is the battle of Stalingrad. The German losses were very heavy and they had to withdraw forces from the West to replace their losses. They were also not able to achieve their objective of accessing the oil fields of Maikop and Grozny to replenish their depleted oil resources which further compounded their logistical issues .
I'm no history buff, but I remember learning that Stalingrad was the turning point in World War Two. For fuck's sake, the Nazis tried invading Russia in the WINTER. Crazy Russians + cold = a bad time.