What Axis defeats in World War 2 led to the most long term damage to the Axis war effort?
Stalingrad and Midway.
Stalingrad - in the broader scope - meant the destruction of a third of the German armed forces, as well as the destruction of her allied armies (Romania, Hungary and Italy's expeditionary corps in USSR). Germany never recovered, but was yet powerful enough to delay defeat a further two years.
Midway meant the destruction of the Japanese surface fleet dominance, a central ability to warfare in the Pacific. Japan also never recovered, but was able to delay defeat with considerable tenacity for years, wholly without prospect of victory.
Stalingrad was a disaster but even though the German Army was bigger and more powerfull in 1943. The Russians that drove the Germans back from Stalingrad were stopped and crushed by Feild marshall Von Mainstien ( Planned the invasion of France) and 6 new SS panzer divisions. Stalin wrote that never had he been so close to defeat. Von Mainstiens attack ground to a halt in the mud but he wanted to begin again a few months later when the ground froze Hitler decided to wait until the following summer so the new panther and Tiger tanks could be used Von Mainstien called this milatary suicide and the Germans were malled at Kurst. It was this battle that they never recoverd from. Shangster