Good old Darius does not deserve to be up there.

His every step was quite clever.
He managed to hire far more Greek mercenaries than the Macedonians (Philip and Alexander), that shows rather competent thinking since it was common knowledge that the Persian infantry was about worth... Well not much. There were the Immotals and the Kardakes and that was it. He obviously didn't trust the grey mass of spearmen, and rightly put them where they belonged, in the rear, to be a sort of human wall.
He is thought to have helped Olympias with the murder of Philip, that in itself is a very clever move. By helping her he would have stirred up things more in Macedon than just outright killing Philip (and causing international outrage).
In his tactical dispositions he did everything that he should.
First he let the very capable Memnon wage a very succesful war of attrition and counterattacks in Asia Minor (why do you think it took so long to get through Asia Minor?), but he was unlucky that Memnon died. Then he launched an offensive in the north that managed to do rather well. But it was called back as Alexander was about to cut it off at the Cilician Gates.

At Issus he massed his cavalry for an overwhelming attack. Right choice!
Had Alexander done the 'right' thing and deployed to meet the threat he would have been rolled over by sheer numbers. Instead the insolent Macedonian dares to a trick and a flanking attack on the cavalry. Unfair to judge him on that.

At Gaugamela he clearly showed that he had learned his lesson.
He made great use of flat terrain to either side, making certain he would outflank his enemy. This usually meant a sure victory. He also employed the usual dense formation breakers, elephants and heavy chariots.
Again Alexander digs deep in his genius and deploys in two lines to deny any rear attacks and has the light infantry deal with the chariots and elephants. While he lures the enemy cavalry on a chase so that he can strike at them when broken.
What could he ever have done against this? Nothing.

Further, and this might not be taken as much of a positive note, but he did try to get a peace going with Alexander, knowing that he couldn't win in battle. It takes a great man to see his own limitations, one of the vitues Alexander for one lacked.

So while Darius was no great leader, and not a superb commander, hewas more than capable, and he was bright. He does not deserve to be on this list.