Declaring war on the USA in 1940
Also invading the Soviet republic
What was Hitler's biggest blunder of all?
The shape of his mustache. What was that man thinking???
I think it was probably over extending his army. Fighting a war of that caliber on multiple fronts usually doesn't end well.
Breaking his pact with Stalin. (Possibly both broke the pact, but if they had remained allies rather than enemies Hitler would have a stronger force for Italy and France)
#1, Operation Barbarossa.
#2, of course, was his failure to invade Britain after Dunkirk. He simply didn't believe how badly defeated the Brits were. An invasion at that time would have been decisive.
Sending kids into war
He killed the guy who killed Hitler.
Thinking that he was a god that was invincible.
No long range bombers and two fronts
I think it was sending a message to Mexico, trying to get them to invade the US. THAT is what really ticked off Americans. We didn't really care about the German UBoats sinking shipping off the east coast ... Probably viewed it as the cost of sending shipments to help England.
Of course ... one could make the case that his BIGGEST blunder was - taking over the German government in the first case.
btw - Have you seen the show "Hunting Hitler"? Alleging that Hitler wasn't killed, and actually escaped afterwards? I know the conspiracy theory has been around since the end of WW2 - but now they've supposedly got declassified documents to back it up.