I think these were the most wasteful aspects:
1) Had Hitler mass produced VWs, Condors, railroad engines, fertilizers, merchant ships, armament, etc, for export in 1939, he would have expanded his economy and industry enormously and been in a much better position to start the war in 1942.
2) Hitler did not use Stalin or Poland to full advantage in 1939. He should have offered Stalin all of Poland (Stalin would have lost a lot of troops, tanks, etc, capturing it). Germany could attack France in 1939, without any British expeditionary force there and even less prepared. Then in 1941 Hitler could have liberated Poland, armed the Poles and used them to invade the USSR.
In 1939, Hitler should have also urged Stalin to invade weak Persia and deprive Britain and France of Persian oil.
3) Hitler wasted his most important resource, his brilliant experts. For example, he got rid of Schacht the brilliant economist who fixed his economy and put the useless Göring in his post. Similarly, he ignored, contradicted or dismissed his field marshals and generals throughout the war. For example he dismissed Guderian, List, Leeb, Manstein, etc, extremely bright strategists and tactitians who had achieved wonders in spite of Hitler's stupidity and constant interferance.
4) Hitler wasted his most important, mobile and expensive armament rather daftly, his airplanes and their crews. He should have been able to defeat France, Belgium and Holland losing a small fraction of the 2,000 planes he lost. He should have eliminated the allied planes before invading, just like the allies did with the Luftwaffe in Sicily, France, Germany, etc, so that the planes concentrate first on destroying enemy planes with few losses and then on supporting the advancing tanks, etc, also with few losses.
Most importantly, Hitler wasted 2,000 planes in the BoB totally pointlessly. These airplanes would have been invaluable first in the Mediterranean and then in the USSR.
The USSR was invaded with ridiculous numbers of planes, after losing 2,000 over France, as many over Britain and several hundred in Poland, Norway, Greece, Yugoslavia, etc, So that the largest country in the world with 28,800 tanks and 20,000 planes was invaded with ridculous numbers of tanks and planes.
Even when attacking the USSR, Hitler did not spend a few days destroying thousands of planes before sending in the army, so that the few German planes had to concentrate on destroying Soviet planes and the tanks received very defficient support during their advance along the huge front, very much weaker than the support afforded Guderian in France. This caused considerable losses in German tanks from the outset, so that by DEcember the Germans were running out of tanks and planes and the offensive ground to a halt all along the front and soon lost a lot of territory gained at a great cost.
5) Hitler also wasted a most valuable thing: time. Instead of going around Sevastopol, Kiev, Smolensk, etc, which were very heavily fortified, Hitler wasted a great many resources capturing these cities, including 8 months of Manstein's time (the first 3 being the most critical time in the war) just capturing Sevastopol, which was arguably the strongest fortification ever built and which could not present any threat to a Blitzkrieg offensive.
By the way, the extremely expensive 800 mm double rail gun used to bombard the fortress cost a fortune to build and transport and required a whole division and its armor piercing shell weighed 7 metric tons, talk about waste. In contrast there were never enough of the useful 88 mm FLAK at the front.