Hitler was a meglamaniacal dictator who had good generals at his disposal and undid virtually everything they'd accomplished militarily.
Alexander strove to defeat the Persian army and then expand and merge the Greek world with the whatever he encountered on his quest. He likely suffered from some sort of depression, certainly, and I'd agree that he benefitted a lot from his father's military creations (most notably the Companion Cavalry and Hypaspists) but he "put the ball into play", so to speak, far better than Phillip ever did.
To be fair to Phillip, he didn't have the chance to quite get the ball rolling. He established (militarily) EVERYTHING for Alexander, Cavalry, Hypaspists and (most importantly) the invention of the Sarissa.
I always like to look upon Alexander's campaigns as explorations. He always sought to keep the status quo with a conquered nation (allowing the satraps to run Persian affairs in his absence and encouraging his soldiers to take Persian brides).
This is shrewd statesmanship, and nothing to do with generalship. Though i mostly attributed it to his strange obsession with the East, mostly to do with his relentless self-aggrandizment.
As far as his Generalship goes, I think it was superior to anything in known history. He simply got the most from what he left Macedon with and used every single aspect from the lowly skirmisher to the mighty Phalanx to it's fullest potential. Hell, his army defeated one of the single greatest weapons known to the world at that time...the War Elephant (though a great cost).
Part of the problem is a lack of primary sources, but by comparing and contrasting later sources, we can attempt to arrive at a picture of the truth.
If you look at the two major accounts, (i need to dig out my thesis for proper references) of the Battle of the River Granicus (his first major encounter in Persia) we find the suggestion that Alexander's over-eagerness almost scuppered him at the start, losing a first battle at night when he attempted to ford the river against a heavily defended, well-trained Persian force.
The next day, when Parmenio (his father's chief general) suggested, he attacked (with tactics) and won.
The accounts of Alexanders battles are littered with references to his superior troops, his enemies poor choice of ground/tactics, and his own reckless charges.
Numbers of enemies (and thus the 'greatness' of his victories) is always over-egged as well.
Alexander won battles on the backs of his exceptional men, not his unexceptional tactics.
He conquered the known world at age 33 for christsakes, you don't just stumble onto that and if you consider it under achieving I'd hate to have to satisfy you.
It is my belief that he did.
Alexander was however a conquerer, and like all conquerers, the one thing he could never gain mastery over was his own lust for glory, therefore he strove ever further eastwards, and would have continued to do so had his men not mutinied.
On his return to persia he actually began to plan the conquest of Arabia, using nearly exclusively persian troops. but he died before it started.