Alexander's army was arguably the best in the ancient world. But the innovations that made it so effective were largely implemented under the reign of Alexander's father Philip II, not under Alexander himself. Likewise, the unification of Greece was completed by the victory at Chaeronea in 338 BC and the establishment of the League of Corinth in 336 BC, both of which were also the achievement of Philip II.
Regarding the victory over Persia, unlike when the Greeks faced them in 490 BC or again in 480 BC, by the time of Alexander the Persians had lost their aura of invincibility having been defeated by Greeks numerous times: at Mycale, at Eurymedon, at Salamis-in-Cyprus, etc. And my understanding is Persia had also been weakened by various internal rebellions in places like Egypt.
Given that the tools which enabled Alexander's conquests -- the Macedonian Army and hegemony over Greece -- were almost entirely an inheritance from his father, and given that the bulk of his conquest was achieved simply by the absorption of the weakened and seemingly already-declining Persian Empire, does history give Alexander too much credit for what he achieved? He is often portrayed as the greatest conqueror of all time, but it seems to me that other conquerors faced more formidable opponents and were more responsible for shaping the circumstances that led to their rise in the first place (ie. Cyrus, Caesar, Genghis Khan, etc.). Or is this unfair?
It's an interesting question.
It's true that Alexander had many of the advantages you describe when he came to the throne. Yet it's unfair to suggest that he'd somehow been handed his victories gift-wrapped by a capable father. That's like saying was Caesar successful because he was Caesar, or because he was a Roman at a time when the Roman legions were the dominant military force in the region. Without a doubt Alexander's accomplishments must be considered mostly his own - he led his men farther than his father had ever dreamed, and inspired in them an extreme loyalty to have brought them as far as India before they turned back.
He was called 'The Great' for a reason. Plutarch tells us that when Caesar thought about Alexander he wept, and when his friends asked why he responded ‘Do you think,’ said he, ‘I have not just cause to weep, when I consider that Alexander at my age had conquered so many nations, and I have all this time done nothing that is memorable?’”
In the famous (and perhaps apocryphal) meeting between Scipio Africanus and Hannibal at the court of Antiochus III, Hannibal ranked Alexander as the greatest general of all time, to which Scipio demured and could find no fault with that even though he wanted to hear his own name. Even if the anecdote isn't strictly speaking true, the fact that it exists at all is representative of how the Ancients themselves thought of Alexander.
But I do think there's something to the idea that Alexander was overrated. To (paraphrase) Caesar again, when he triumphed over Pharnaces of Pontus (and uttered the 'Veni Vidi Vici') he supposedly also commented that Pompey was fortunate to have made his name fighting against 'these people' - suggesting that much of Pompey's fame had been won fighting weak enemies whereas Caesar had made his name in the Gallic Wars. Although this is extremely unfair to Pompey it has a spark of truth to it and to me rings clear in relation to Alexander as well.
Ultimately Alexander's Persian counterpart, Darius, was a weak ruler not up to the challenge of ruling his empire, or fighting a major war against such an energetic opponent. Alexander also had the advantage of a well trained and well blooded army with a new and modern way of making war while the Persians still relied largely on levies of spearmen and contributions from satrapies. In this sense, I see a structural forces at play that undermined the Persians and significantly favored the Macedonians. Yet at the same time, it was Alexander who had the vision and wherewithal to use these advantages, and see it through to the dismemberment of the entire Persian Empire. And it was Darius who couldn't recognize or at least couldn't act to correct the weaknesses of his country and army.
My verdict: Overrated, sure. But I believe Alexander is well deserving of being called The Great, just not The Greatest.