I'm not a flaired user on this subreddit, but I'll give you the information I have:

1. Hitler insisted on the development of dive-bombers instead of large, level-bombers. While this significantly increased the Luftwaffe's ability to support Blitzkrieg warfare, it put the German airforce at a severe disadvantage when it came to the Battle of Britain. Stuka dive-bombers had limited range and bomb capacity and were not well suited to strategic warfare. One can argue about whether switching focus to level-bombing would have then reduced the Germans' ability to fight their Blitzkriegs, but considering how the Allies managed to do the exact same thing to the Germans in their counterattacks without a dive-bomber heavy air force, and further considering Germany's failure to put the UK out of the war before the Americans and Russians joined, I think this could be considered a blunder.

2. Hitler ended the Battle of Britain too early. Though both sides were taking enormous casualties, Britain was cracking at a far faster rate than Germany. Had Hitler kept the pressure up against the UK, even with their radar advantages, Britain could not have kept up with them. Giving Britain a break allowed them to regain their strength and continue resistance.

3. Hitler supported Italy's invasion of Albania, thus delaying the start of Operation Barbarossa (the invasion of Russia). Albania was not significant to the war in any way, except as a side-show for Mussolini's Roman Empire daydreams. Hitler supported it anyway, delaying the start of Operation Barbarossa by a few critical months. Bad weather and stiffening Russian resistance stopped the Germans just short of Moscow in 1941. Had the Wehrmacht been given a few extra months of good weather, I think it could be argued that Moscow would have fallen, with disastrous consequences for the USSR's ability to resist the invasion.

4. Hitler declared war on the United States. Under the terms of the mutual protection pact between Germany and Japan, Germany was only obligated to militarily assist Japan if Japan was attacked, not if Japan was the aggressor. Technically, Hitler could have left Japan to deal with its own war. While this would have been extremely bad for Japan, it could have kept the isolationist United States from getting involved in Europe (more than it already was with Lend-Lease and the basically undeclared war in the Atlantic) allowing them to come to some sort of peace agreement with the Russians outside unconditional surrender.

5. Hitler insisted on defending every speck of ground his armies had conquered. His refusal to even contemplate retreat or even strategic withdrawal was what lead directly to disaster at Stalingrad, and allowed the Russians to continue to inflict serious casualties on the Wehrmacht afterward. In a sense, the war was lost once Russia was allowed to regain its footing when the Germans failed to take Moscow. However, Germany certainly could have held Russia out of Germany and even Poland if the Wehrmacht had been allowed to fight intelligently in defense.

6. The Battle of the Bulge. When the Wehrmacht should have been concentrating on retreating to a defensible perimeter inside of Germany, Hitler instead threw the last of his army's strength into a ludicrously unlikely to succeed counter attack. There was not even enough gas in the army's reserves to support the entire offensive, and in fact the plan counted on captured Allied gasoline to even keep the offensive going. As was almost inevitable, the offensive petered out before it could achieve its objectives, and the Allies rolled on into Germany all the easier.

Those are the big ones that stand out to me, without going so far as naming mistakes like "invading Russia in the first place" or "starting the war at all." To me, those things are like saying ,"what if Hitler wasn't Hitler?"