There are probably three key junctures here:
June-December 1940: The success of the Dunkirk evacuation and the failure of the Battle of Britain (thus making it certain that Britain would continue to fight) meant that Hitler lost his best opportunity to reorder the European state system along the lines that he wished.
June-December 1941: The failure of Barbarossa and the entry of the Americans into the war made it very unlikely that the Germans would ‘win’ the war.
October 1942-July 1943: The disaster in Stalingrad, the loss of the war in North Africa and subsequent invasion of Italy helped lock in not simply a German defeat, but the catastrophe of 1945.
This, with the caveat that the last window should be June-September 42. By October it was clear Fall Blau and the attempt to seize Leningrad were not going to work (even if Operation Uranus was - somehow - contained), the Afrika Korps was in an unwinnable position at El Alamein with Operation Torch incoming, the window to seize Malta had gone, the Japanese being mauled at Midway lessened their ability to keep the US busy for longer, and so on.
For Barbarossa, the failure to seize Moscow is often cited as a key turning point... but it's not clear that the Germans could have taken it at any point. It was always logistically out of reach for an absurdly overstreched and mauled Army Group Center, and even if the Germans had made it there and the government evacuated... it could have easily degenerated into a bloody siege and an early Stalingrad at the first opportunity for a Soviet counteroffensive. And while losing Moscow would have seriously hampered the Soviet war effort, it's not by any means certain - probably the opposite - that Soviet morale would have finally cracked.
If we are going that far, might as well go to the Night of the Long Knives where Hitler purged all the competent officers, making the German military rely on aristocrats who were incompetent at their jobs.
With respect, I don't see how that's accurate. The only generals killed there were - if memory serves right - Schleicher and Bredow, and both were political generals.
And while the competence of several Wehrmacht officers has been overrated due to legend (Rommel), covering up for their own blunders by blaming Hitler (Halder), and recent re-evaluation of their limited grasp of logistics and strategy (compared to an overreliance on tactics) it's not like they were a gang of incompetent aristocrats either.
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