Hitler's decisions are the top I'd say. To start off...If the atomsphere was right and he listened to his officers he could've easily waited a few years and built up his army to the corp that would be needed to fight a large scale conflict with many countries involed. You'd see Jet powered planes bombing Warsaw, Riflemen carrying high caliber STG44 variants, huge armadas of King Tigers ( other experimental tanks ) roaming the land, a lot of rockets, and a lot of men in uniform. This would take a few years to achieve yes, but don't forget the fact that Germany was trying to "do" what I just said in the end days of the war,etc. And of course, this was impossible since industrial nazi-germany was pretty much bombed out by then. But at the beginning and slow burning parts of the war industrial nazi-germany was in it's prime.
Commanding the war personally. Political leaders should focus on politics. The more deeply involved politicians get in the running of a military operation the more screwed up that operation usually gets.
Another blunder was in using Methamphetamines. A lot of the flawed decision-making going on outright delusion was probably a result of the Meth, or at least was aggravated by it.
Blitzing all of the way to the Dunkirk beachead would certainly have improved his situation, but wouldn't have been a knockout blow. It would have cost the British a lot of experienced troops but they could have come back from it, given time.
Keeping the pressure on the front line airfields and radar stations along the south coast of England during the Battle of Britain would have made a huge difference, perhaps enough to be decisive. Especially if combined with crushing the beachead at dunkirk, England could potentially have been taken out of the war.
Not concentrating on North Africa and the mediteranean in 1941 was another big mistake. This of course assumes that he was unable to invade England successfully. Taking control of this area would have done three things. It would have secured vital petrolium reserves which would have meant plenty of fuel for combat operations, plenty of petrolium for manufacturing fertilizer and for making propellants too. Securing the mediteranean, especially Gibraltar and the Suez Canal would have further isolated England from her colonies and would have helped out the Italian fleet. Also it would have allowed Germany and Japan to join forces. Together they could have shut down the lend-lease pipeline to the Soviet Union with ease and begun the process of crushing the Soviets. A small note: although it is huge the Soviet Union was never able to feed itself or exploit its resources efficiently. Unless significant reforms were made, it would have collapsed. Even with massive ongoing economic assistance from the west it eventually collapsed anyway.
The lack of petrolium may also explain the German failure to deploy the Me 262 earlier, since jet engines burn a lot more fuel than proeller planes and would have made supply issues more difficult. Also the critical shortage of tungsten which is needed to withstand the high temperatures of a jet engine.
One of Hitlers big blunders was that he had developed a contemptuous attitude toward the Soviets based on reports from German "volunteers" serving with Finnland during the Winter War between the Finns and Soviets between '39 and 40. After their humiliation at the hands of the Finns, Hitler assumed that the Soviets could be crushed without difficulty.
Ordering Gundarian to surround and mop up enemy troops instead of driving straight for Moscow was a huge blunder. I don't know if the Soviets would have surrendered even if Moscow had been taken, but it was the Germans' best hope for a quick end to the war.
Hitler made many more mistakes of course:
http://www.faqfarm.com/History/WWII/8427