In case you ask me.
When UK and France declared war , Hitler could have haltet his troops on Germany’s original 1914 borders and stated that he had no more claims, and that the treaties of Versailles and the spirit of Wilson was fulfilled. Then UK and France would be short of any casus belli, and there would be no world war, and Hitler would be considered one of the greatest statesmen of the century because he had restored Germany’s originale borders, not to mention that he lifted Germany out of the depression.
But since he kept on, the best thing he could do different, was better cooperation with his allies and his generals.
The treaty with Stalin was clever, the Poland campaign was a success, the attack on Norway to secure the steel trade and treathen the North Sea was a success, and the initial attack on France was a success. But by now Hitler started to mess it up, and he was responsible for the miracle of Dunquirk. If he had trusted his generals, he could have got 350 000 british hostages to enforce a peace treaty.
But from this point his Italian allied startet to mess things up in Balkan and North Africa. Mussolini attacked Egypt and Greece without telling Hitler, and failed in both places. Hitler could have ignored it, and saved every man for Barbarossa, but then the oil fields in Ploesti would have been vulnerable. Better cooperation with Italy could have avoided the attacks on Egypt and Greece, and made for an ultimate Barbarossa campaign. Or Hitler could have put of Barbarossa, and used all available resources to conquer the rich oil fields in the Middle East, closed the Sues canal, allied with Turkey, and kicked the Brits out of North Africa and the Med. Now Germany would have a great template for a 1942 Russian campaign.
Better cooperation with Japan could made a successful Barbarossa. If Japan attacked Russia simultanesly, and Hitler let Army Group Center go straight for Moscow, then Moscow would have been conqered. In our real timeline, Moscow was saved by the siberian troops and the non-aggression pact with Japan. As it turned out, the Japanese attack on Pearl Harbour did not help Germany, it just brought giant military industrial complex USA into the war, and from that point the outcome was pretty much set.